The Project Gutenberg eBook ofA-B-C of Electricity

The Project Gutenberg eBook ofA-B-C of ElectricityThis ebook is for the use of anyone anywhere in the United States and most other parts of the world at no cost and with almost no restrictions whatsoever. You may copy it, give it away or re-use it under the terms of the Project Gutenberg License included with this ebook or online atwww.gutenberg.org. If you are not located in the United States, you will have to check the laws of the country where you are located before using this eBook.Title: A-B-C of ElectricityAuthor: Wm. H. MeadowcroftRelease date: June 13, 2014 [eBook #45955]Most recently updated: October 24, 2024Language: EnglishCredits: Produced by Giovanni Fini and the Online DistributedProofreading Team at http://www.pgdp.net (This file wasproduced from images generously made available by TheInternet Archive)*** START OF THE PROJECT GUTENBERG EBOOK A-B-C OF ELECTRICITY ***

This ebook is for the use of anyone anywhere in the United States and most other parts of the world at no cost and with almost no restrictions whatsoever. You may copy it, give it away or re-use it under the terms of the Project Gutenberg License included with this ebook or online atwww.gutenberg.org. If you are not located in the United States, you will have to check the laws of the country where you are located before using this eBook.

Title: A-B-C of ElectricityAuthor: Wm. H. MeadowcroftRelease date: June 13, 2014 [eBook #45955]Most recently updated: October 24, 2024Language: EnglishCredits: Produced by Giovanni Fini and the Online DistributedProofreading Team at http://www.pgdp.net (This file wasproduced from images generously made available by TheInternet Archive)

Title: A-B-C of Electricity

Author: Wm. H. Meadowcroft

Release date: June 13, 2014 [eBook #45955]Most recently updated: October 24, 2024

Language: English

Credits: Produced by Giovanni Fini and the Online DistributedProofreading Team at http://www.pgdp.net (This file wasproduced from images generously made available by TheInternet Archive)

*** START OF THE PROJECT GUTENBERG EBOOK A-B-C OF ELECTRICITY ***

FIRST DIRECT-CONNECTED ELECTRIC GENERATOR UNIT OF LARGE CAPACITY EVER CONSTRUCTED UP TO THE TIME IT WAS MADE BY THOMAS A. EDISON IN JUNE, 1881. CAPACITY, 1200 INCANDESCENT LAMPS OF 16 CANDLE-POWER EACHA-B-COFELECTRICITYBYWILLIAM H. MEADOWCROFTHARPER & BROTHERS PUBLISHERSNEW YORK & LONDONA-B-C of ElectricityCOPYRIGHT, 1888, 1909, BY WILLIAM H. MEADOWCROFTCOPYRIGHT, 1915, BY HARPER & BROTHERSPRINTED IN THE UNITED STATES OF AMERICAPUBLISHED MAY, 1915

BYWILLIAM H. MEADOWCROFT

HARPER & BROTHERS PUBLISHERS

NEW YORK & LONDON

A-B-C of Electricity

From the Laboratory of Thomas A. EdisonOrange, N. J.Mr. W. H. Meadowcroft,New York City.DEAR SIR:I have read the MS. of your "A-B-C of Electricity," and find that the statements you have made therein are correct. Your treatment of the subject, and arrangement of the matter, have impressed me favorably.Yours truly,THOS. A. EDISON

From the Laboratory of Thomas A. Edison

Orange, N. J.

Mr. W. H. Meadowcroft,

New York City.

DEAR SIR:

I have read the MS. of your "A-B-C of Electricity," and find that the statements you have made therein are correct. Your treatment of the subject, and arrangement of the matter, have impressed me favorably.

Yours truly,

THOS. A. EDISON

CONTENTSCHAP.PAGEIntroduction To New EditionviiiPrefacexI.1II.Definitions3III.Magnetism16IV.The Telegraph23V.Wireless Telegraphy33VI.The Telephone40VII.Electric Light54VIII.Electric Power87IX.Batteries95X.Conclusion127

INTRODUCTION TO NEW EDITIONThe favor with which this book has been received has brought about the preparation of this new edition. The present volume has been enlarged by the addition of certain new material and it has been entirely reset. Some new illustrations have been made, and in its new dress the book, it is hoped, will be found to afford an even larger measure of usefulness. The principles of the science remain the same, but the author is glad of the opportunity to note certain developments in their application.W. H. M.Edison Laboratory,April, 1915.

The favor with which this book has been received has brought about the preparation of this new edition. The present volume has been enlarged by the addition of certain new material and it has been entirely reset. Some new illustrations have been made, and in its new dress the book, it is hoped, will be found to afford an even larger measure of usefulness. The principles of the science remain the same, but the author is glad of the opportunity to note certain developments in their application.

W. H. M.

Edison Laboratory,April, 1915.

PREFACEWhile there is no lack of most excellent text-books for the study of those branches of Electricity which are above the elementary stage, there is a decided need of text-books which shall explain, in simple language, to young people of, say, fourteen years and upward, a general outline of the science, as well as the ground-work of those electrical inventions which are to-day of such vast commercial importance.There is also a need for such a book among a large part of the adult population, for the reason that there have been great and radical changes in this science since the time they completed their studies, and they have not the time to follow up the subject in the advanced books.As instances of those changes just spoken of, the electric light, telephone, and storage batteries may be mentioned, which have been developed during the last ten or twelve years, with the result of adding very manyfeatures that were entirely new to electricians.With these ideas in view I have prepared this little volume. It is not intended, in the slightest degree, to be put forward as a scientific work, but it will probably give to many the information they desire without requiring too great a research into books which treat more extensively and deeply of this subject.W. H. M.

While there is no lack of most excellent text-books for the study of those branches of Electricity which are above the elementary stage, there is a decided need of text-books which shall explain, in simple language, to young people of, say, fourteen years and upward, a general outline of the science, as well as the ground-work of those electrical inventions which are to-day of such vast commercial importance.

There is also a need for such a book among a large part of the adult population, for the reason that there have been great and radical changes in this science since the time they completed their studies, and they have not the time to follow up the subject in the advanced books.

As instances of those changes just spoken of, the electric light, telephone, and storage batteries may be mentioned, which have been developed during the last ten or twelve years, with the result of adding very manyfeatures that were entirely new to electricians.

With these ideas in view I have prepared this little volume. It is not intended, in the slightest degree, to be put forward as a scientific work, but it will probably give to many the information they desire without requiring too great a research into books which treat more extensively and deeply of this subject.

W. H. M.

A-B-C OF ELECTRICITYA-B-C OF ELECTRICITYIWe now obtain so many of our comforts and conveniences by the use of electricity that all young people ought to learn something of this wonderful force, in order to understand some of the principles which are brought into practice.You all know that we have the telegraph, the telephone, the electric light, electric motors on street-cars, electric bells, etc., besides many other conveniences which the use of electricity gives us.Every one knows that, by the laws of multiplication, twice two makes four, and that twice two can never make anything but four. Well, these useful inventions have been made by applying thelaws of electricityin certain ways, just as well known, so as to enable us to send in a few moments a message to ourabsent friends at any distance, to speak with them at a great distance, to light our houses and streets with electric light, and to do many other useful things with quickness and ease.But you must remember that we do not know what electricity itself really is. We only know how to produce it by certain methods, and we also know what we can do with it when we have obtained it.In this little book we will try to explain the various ways by which electricity is obtained, and how it is applied to produce the useful results that we see around us.We will try and make this explanation such that it will encourage many of you to study this very important and interesting subject more deeply.In the advanced books on electricity there are many technical terms which are somewhat difficult to understand, but in this book it will only be necessary to use a few of the more simple ones, which it will be well for you to learn and understand before going further.

A-B-C OF ELECTRICITY

We now obtain so many of our comforts and conveniences by the use of electricity that all young people ought to learn something of this wonderful force, in order to understand some of the principles which are brought into practice.

You all know that we have the telegraph, the telephone, the electric light, electric motors on street-cars, electric bells, etc., besides many other conveniences which the use of electricity gives us.

Every one knows that, by the laws of multiplication, twice two makes four, and that twice two can never make anything but four. Well, these useful inventions have been made by applying thelaws of electricityin certain ways, just as well known, so as to enable us to send in a few moments a message to ourabsent friends at any distance, to speak with them at a great distance, to light our houses and streets with electric light, and to do many other useful things with quickness and ease.

But you must remember that we do not know what electricity itself really is. We only know how to produce it by certain methods, and we also know what we can do with it when we have obtained it.

In this little book we will try to explain the various ways by which electricity is obtained, and how it is applied to produce the useful results that we see around us.

We will try and make this explanation such that it will encourage many of you to study this very important and interesting subject more deeply.

In the advanced books on electricity there are many technical terms which are somewhat difficult to understand, but in this book it will only be necessary to use a few of the more simple ones, which it will be well for you to learn and understand before going further.

IIDEFINITIONSThe three measurements most frequently used in electricity areThe Volt,The Ampère,The Ohm.We will explain these in their order.Fig. 1The Volt.—This term may be better understood by making a comparison with something you all know of. Suppose we have a tank containing one hundred gallons of water, and we want to discharge it through a half-inch pipe at the bottom of the tank. Suppose, further, that we wanted to make the water spout upward,and for this purpose the pipe was bent upward as in Fig. 1.If you opened the tap the water would spout out and upward as in Fig. 1.Fig. 2The cause of its spouting upward would be theweightorpressureof the water in the tank. This pressure is reckoned as so manypoundsto the square inch of water.Now, if the tank were placed on the roof of the house and the pipe brought to the ground as shown in Fig. 2, the water would spout up very much higher, because there would bemany more poundsof pressure on account of the height of the pipe.So, you see, the force or pressure of water is measured in pounds, and, therefore, a pound is the unit of pressure, or force, of water. Now, in electricity the unit of pressure, or force, is called a volt.This word "volt" does not mean anyweight, as the word "pound" weight does. You all know that if you have a pound of water you must have something to hold it, because it has weight, and, consequently, occupies some space. Butelectricity itself has no weightand therefore cannot occupy any space.When we desire to carry water into a house or other building we do so by means of hollow pipes, which are usually made of iron. This is the way that water is brought into houses in cities and towns, so that it may be drawn and used in any part of a dwelling. Now, the principal supply usually comes from a reservoir which is placed up on high ground so as to give the necessary pounds of pressure to force the water up to the upper part of the houses. If some arrangement of this kind were not made we could get no water in our bedrooms, because, as you know, water will not rise above its own level unless by force.The water cannot escape as long as there are no holes or leaks in the iron pipes, but if there should be the slightest crevice in them the water will run out.In electricity we find similar effects.The electricity is carried into houses by means of wires which are covered, orinsulated,with various substances, such, for instance, as rubber. Just as the iron of the pipes prevents the water from escaping, the insulation of the wire prevents the escape of the electricity.Now, if we were to cause the pounds of pressure of water, in pipes of ordinary thickness, to be very greatly increased, the pipes could not stand the strain and would burst and the water escape. So it is with electricity. If there were too many volts of pressure the insulation would not be sufficient to hold it and the electricity would escape through the covering, or insulation, of the wire.It is a simple and easy matter to stop the flow of water from an ordinary faucet by placing your finger over the opening. As the water cannot then flow, your finger is what we will call a non-conductor and the water will be retained in the pipe.We have just the same effects in electricity. If we place some substance which is practically a non-conductor, or insulator, such as rubber, around an electric wire, or in the path of an electric current, the electricity, acted upon by the volts of pressure, cannot escape, because the insulation keeps it from doing so, just as the iron of the pipe keeps thewater from escaping. Thus, you see, the volt does not itself represent electricity, but only the pressure which forces it through the wire.There are other words and expressions in electricity which are sometimes used in connection with the word "volt." These words are "pressure" and "intensity." We might say, for instance, that a certain dynamo machine had an electromotive force of 110 volts; or that the intensity of a cell of a battery was 2 volts, etc.We might mention, as another analogy, the pressure of steam in a boiler, which is measured or calculated in pounds, just as the pressure of water is measured. So, we might say that 100 pounds steam pressure used through the medium of a steam-engine to drive a dynamo could thus be changed to electricity at 100 volts pressure.The Ampère.—Now, in comparing the pounds pressure of water with the volts of pressure of electricity we used as an illustration a tank of water containing 100 gallons, and we saw that this water had a downward force or pressure in pounds. Let us now see what this pressure was acting upon.It was forcing the quantity of water to spout upward through the end of the pipe.Now, as the quantity of water was 100 gallons, it could not all be forced at once out of the end of the pipe. The pounds pressure of water acting on the 100 gallons would force it out at acertain rate, which, let us say, would be one gallon per minute.This would be therate of the flowof water out of the tank.Thus, you see, we find a second measurement to be considered in discharging the water-tank. The first was the force, or pounds of pressure, and the second therateat which the quantity of water was being discharged per minute by that pressure.This second measurement teaches us that acertain quantitywill pass out of the pipe in acertain timeif the pressure is steady, such quantity depending, of course, on the size or friction resistance of the pipe.In electricity the volts of pressure act so as to force the quantity of current toflow through the wires at a certain rateper second, and the rate at which it flows is measured in ampères. For instance, let us suppose that an electric lamp required a pressure of 100 volts and a current of one ampère to light it up, we should have to supply a current of electricity flowing at the rate of one ampère, acted upon by an electromotive force of 100 volts.You will see, therefore, that while the volt does not represent any electricity, but only its pressure, the ampère represents therate of flowof the current itself.You should remember that there are several words sometimes used in connection with the word "ampère"—for instance, we might say that a lamp required a "current" of one ampère or that a dynamo would give a "quantity" of 20 ampères.The Ohm.—You have learned that thepressurewould discharge thequantityof water at a certain rate through the pipe. Now, suppose we were to fixtwodischarge-pipes to the tank, the water would run away very much quicker, would it not? If we try to find a reason for this, we shall see that a pipe can only, at a given pressure, admit so much water through it at a time.Therefore, you see, this pipe would present a certain amount ofresistanceto the passage of the total quantity of water, and would only allow a limited quantity at once to go through. But, if we were to attach two or more pipes to the tank, or one large pipe, we should make it easier for the water to flow, and, therefore, the total amount of resistance to the passage of the water would be very much less, and the tank would quickly be emptied.Now, as you already know, water has substance and weight and therefore occupies some space, but electricity has neither substance nor weight, and therefore cannot occupy any space; consequently, to carry electricity from one place to another we do not need to use a pipe, which is hollow, but we use a solid wire.These solid wires have a certain amount ofresistanceto the passage of the electricity, just as the water-pipe has to the water, and (as it is in the case of the water) the effect of the resistance to the passage of electricity is greater if you pass a larger quantity through than a smaller quantity.If you wanted to carry a quantity of electricity to a certain distance, and for that purpose used a wire, there would be a certain amount of resistance in that wire to the passage of the current through it; but if you used two or more wires of the same size, or one large wire, the resistance would be very much less and the current would flow more easily.Suppose that, instead of emptying the water-tank from the roof through the pipe, we had just turned the tank over and let the water all pour out at once down to the ground. That would dispose of the water very quickly and by a short way, would itnot? That is very easy to be seen, because there would beno resistanceto its passage to the ground.Well, suppose we had an electric battery giving a certain quantity of current, say five ampères, and we should take a large wire that would offer no resistance to that quantity and put it from one side of the battery to the other, a large current would flow at once and tend to exhaust the battery. This is called ashort circuitbecause there is little or no resistance, and it provides the current with an easy path to escape. Remember this, thatelectricity always takes the easiest path. It will take as many paths as are offered, but the largest quantity will always take the easiest.As the subject of resistance is one of the most important in electricity, we will give you one more example, because if you can obtain a good understanding of this principle it will help you to comprehend the whole subject more easily in your future studies.We started by comparison with a tank holding 100 gallons of water, discharging through a half-inch pipe, and showed you that the pounds of pressure would force the quantity of gallons through the pipe. When the tap was first opened the water would spout up very high, but as the water in thetank became lower the pressure would be less, and, consequently, the water would not spout so high.So, if it were desired to keep the water spouting up to the height it started with, we should have to keep the tank full, so as to have the same pounds of pressure all the time. But, if we wanted the water to spout still higher we should have to use other means, such as a force-pump, to obtain a greater pressure.Now, if we should use too many pounds pressure it would force the quantity of water more rapidly through the pipe and would cause the water to become heated because of the resistance of the pipe to the passage of that quantity acted upon by so great a pressure.This is just the same in electricity, except that the wire itself would become heated, some of the electricity being turned into heat and lost. If a wire were too small for the volts pressure and ampères of current of electricity the resistance of such wire would be overcome, and it would become red-hot and perhaps melt. Electricians are therefore very careful to calculate the resistance of the wires they use before putting them up, especially when they are for electric lighting, inorder to make allowances for the ampères of current to flow through them, so that but little of the electricity will be turned into heat and thus rendered useless for their purpose.The unit of resistance is called theohm(pronounced like "home" without the "h").All wires have a certain resistance per foot, according to the nature of the metal used and the size of the wire—that is to say, the finer the wire the greater number of ohms resistance it has to the foot.Water and electricity flow under very similar conditions—that is to say, each of them must have a channel, or conductor, and each of them requires pressure to force it onward. Water, however, being a tangible substance, requires a hollow conductor; while electricity, being intangible, will flow through a solid conductor. The iron of the water-pipe and the insulation of the electric wire serve the same purpose—namely, that of serving to prevent escape by reason of the pressure exerted.There is another term which should be mentioned in connection with resistance, as they are closely related, and that isopposition. There is no general electrical term of this name, but, as it will be most easily understoodfrom the meaning of the word itself, we have used it.Let us give an example of what opposition would mean if applied to water. Probably every one knows that a water-wheel is a wheel having large blades, or "paddles," around its circumference.When the water, in trying to force its passage, rushes against one of these paddles it meets with its opposition, but overcomes it by pushing the paddle away. This brings around more opposition in the shape of another paddle, which the water also pushes away. And so this goes on, the water overcoming this opposition and turning the wheel around, by which means we can get water to do useful work for us.You must remember, however, that it is only by putting opposition in the path of a pressure and quantity of water that we can get this work.The same principle holds good in electricity. We make electricity in different ways, and in order to obtain useful work we put in its path the instruments, lamps, or machines which offer the proper amount of resistance, or opposition, to its passage, and thus obtain from this wonderful agent the work we desire to have done.You have learned that three important measurements in electricity are as follows:Thevoltis the practical unit of measurement ofpressure;Theampèreis the practical unit of measurement of therate of flow; andTheohmis the practical unit of measurement ofresistance.

DEFINITIONS

The three measurements most frequently used in electricity are

The Volt,The Ampère,The Ohm.

We will explain these in their order.

Fig. 1

The Volt.—This term may be better understood by making a comparison with something you all know of. Suppose we have a tank containing one hundred gallons of water, and we want to discharge it through a half-inch pipe at the bottom of the tank. Suppose, further, that we wanted to make the water spout upward,and for this purpose the pipe was bent upward as in Fig. 1.

If you opened the tap the water would spout out and upward as in Fig. 1.

Fig. 2

The cause of its spouting upward would be theweightorpressureof the water in the tank. This pressure is reckoned as so manypoundsto the square inch of water.

Now, if the tank were placed on the roof of the house and the pipe brought to the ground as shown in Fig. 2, the water would spout up very much higher, because there would bemany more poundsof pressure on account of the height of the pipe.

So, you see, the force or pressure of water is measured in pounds, and, therefore, a pound is the unit of pressure, or force, of water. Now, in electricity the unit of pressure, or force, is called a volt.

This word "volt" does not mean anyweight, as the word "pound" weight does. You all know that if you have a pound of water you must have something to hold it, because it has weight, and, consequently, occupies some space. Butelectricity itself has no weightand therefore cannot occupy any space.

When we desire to carry water into a house or other building we do so by means of hollow pipes, which are usually made of iron. This is the way that water is brought into houses in cities and towns, so that it may be drawn and used in any part of a dwelling. Now, the principal supply usually comes from a reservoir which is placed up on high ground so as to give the necessary pounds of pressure to force the water up to the upper part of the houses. If some arrangement of this kind were not made we could get no water in our bedrooms, because, as you know, water will not rise above its own level unless by force.

The water cannot escape as long as there are no holes or leaks in the iron pipes, but if there should be the slightest crevice in them the water will run out.

In electricity we find similar effects.

The electricity is carried into houses by means of wires which are covered, orinsulated,with various substances, such, for instance, as rubber. Just as the iron of the pipes prevents the water from escaping, the insulation of the wire prevents the escape of the electricity.

Now, if we were to cause the pounds of pressure of water, in pipes of ordinary thickness, to be very greatly increased, the pipes could not stand the strain and would burst and the water escape. So it is with electricity. If there were too many volts of pressure the insulation would not be sufficient to hold it and the electricity would escape through the covering, or insulation, of the wire.

It is a simple and easy matter to stop the flow of water from an ordinary faucet by placing your finger over the opening. As the water cannot then flow, your finger is what we will call a non-conductor and the water will be retained in the pipe.

We have just the same effects in electricity. If we place some substance which is practically a non-conductor, or insulator, such as rubber, around an electric wire, or in the path of an electric current, the electricity, acted upon by the volts of pressure, cannot escape, because the insulation keeps it from doing so, just as the iron of the pipe keeps thewater from escaping. Thus, you see, the volt does not itself represent electricity, but only the pressure which forces it through the wire.

There are other words and expressions in electricity which are sometimes used in connection with the word "volt." These words are "pressure" and "intensity." We might say, for instance, that a certain dynamo machine had an electromotive force of 110 volts; or that the intensity of a cell of a battery was 2 volts, etc.

We might mention, as another analogy, the pressure of steam in a boiler, which is measured or calculated in pounds, just as the pressure of water is measured. So, we might say that 100 pounds steam pressure used through the medium of a steam-engine to drive a dynamo could thus be changed to electricity at 100 volts pressure.

The Ampère.—Now, in comparing the pounds pressure of water with the volts of pressure of electricity we used as an illustration a tank of water containing 100 gallons, and we saw that this water had a downward force or pressure in pounds. Let us now see what this pressure was acting upon.

It was forcing the quantity of water to spout upward through the end of the pipe.Now, as the quantity of water was 100 gallons, it could not all be forced at once out of the end of the pipe. The pounds pressure of water acting on the 100 gallons would force it out at acertain rate, which, let us say, would be one gallon per minute.

This would be therate of the flowof water out of the tank.

Thus, you see, we find a second measurement to be considered in discharging the water-tank. The first was the force, or pounds of pressure, and the second therateat which the quantity of water was being discharged per minute by that pressure.

This second measurement teaches us that acertain quantitywill pass out of the pipe in acertain timeif the pressure is steady, such quantity depending, of course, on the size or friction resistance of the pipe.

In electricity the volts of pressure act so as to force the quantity of current toflow through the wires at a certain rateper second, and the rate at which it flows is measured in ampères. For instance, let us suppose that an electric lamp required a pressure of 100 volts and a current of one ampère to light it up, we should have to supply a current of electricity flowing at the rate of one ampère, acted upon by an electromotive force of 100 volts.

You will see, therefore, that while the volt does not represent any electricity, but only its pressure, the ampère represents therate of flowof the current itself.

You should remember that there are several words sometimes used in connection with the word "ampère"—for instance, we might say that a lamp required a "current" of one ampère or that a dynamo would give a "quantity" of 20 ampères.

The Ohm.—You have learned that thepressurewould discharge thequantityof water at a certain rate through the pipe. Now, suppose we were to fixtwodischarge-pipes to the tank, the water would run away very much quicker, would it not? If we try to find a reason for this, we shall see that a pipe can only, at a given pressure, admit so much water through it at a time.

Therefore, you see, this pipe would present a certain amount ofresistanceto the passage of the total quantity of water, and would only allow a limited quantity at once to go through. But, if we were to attach two or more pipes to the tank, or one large pipe, we should make it easier for the water to flow, and, therefore, the total amount of resistance to the passage of the water would be very much less, and the tank would quickly be emptied.

Now, as you already know, water has substance and weight and therefore occupies some space, but electricity has neither substance nor weight, and therefore cannot occupy any space; consequently, to carry electricity from one place to another we do not need to use a pipe, which is hollow, but we use a solid wire.

These solid wires have a certain amount ofresistanceto the passage of the electricity, just as the water-pipe has to the water, and (as it is in the case of the water) the effect of the resistance to the passage of electricity is greater if you pass a larger quantity through than a smaller quantity.

If you wanted to carry a quantity of electricity to a certain distance, and for that purpose used a wire, there would be a certain amount of resistance in that wire to the passage of the current through it; but if you used two or more wires of the same size, or one large wire, the resistance would be very much less and the current would flow more easily.

Suppose that, instead of emptying the water-tank from the roof through the pipe, we had just turned the tank over and let the water all pour out at once down to the ground. That would dispose of the water very quickly and by a short way, would itnot? That is very easy to be seen, because there would beno resistanceto its passage to the ground.

Well, suppose we had an electric battery giving a certain quantity of current, say five ampères, and we should take a large wire that would offer no resistance to that quantity and put it from one side of the battery to the other, a large current would flow at once and tend to exhaust the battery. This is called ashort circuitbecause there is little or no resistance, and it provides the current with an easy path to escape. Remember this, thatelectricity always takes the easiest path. It will take as many paths as are offered, but the largest quantity will always take the easiest.

As the subject of resistance is one of the most important in electricity, we will give you one more example, because if you can obtain a good understanding of this principle it will help you to comprehend the whole subject more easily in your future studies.

We started by comparison with a tank holding 100 gallons of water, discharging through a half-inch pipe, and showed you that the pounds of pressure would force the quantity of gallons through the pipe. When the tap was first opened the water would spout up very high, but as the water in thetank became lower the pressure would be less, and, consequently, the water would not spout so high.

So, if it were desired to keep the water spouting up to the height it started with, we should have to keep the tank full, so as to have the same pounds of pressure all the time. But, if we wanted the water to spout still higher we should have to use other means, such as a force-pump, to obtain a greater pressure.

Now, if we should use too many pounds pressure it would force the quantity of water more rapidly through the pipe and would cause the water to become heated because of the resistance of the pipe to the passage of that quantity acted upon by so great a pressure.

This is just the same in electricity, except that the wire itself would become heated, some of the electricity being turned into heat and lost. If a wire were too small for the volts pressure and ampères of current of electricity the resistance of such wire would be overcome, and it would become red-hot and perhaps melt. Electricians are therefore very careful to calculate the resistance of the wires they use before putting them up, especially when they are for electric lighting, inorder to make allowances for the ampères of current to flow through them, so that but little of the electricity will be turned into heat and thus rendered useless for their purpose.

The unit of resistance is called theohm(pronounced like "home" without the "h").

All wires have a certain resistance per foot, according to the nature of the metal used and the size of the wire—that is to say, the finer the wire the greater number of ohms resistance it has to the foot.

Water and electricity flow under very similar conditions—that is to say, each of them must have a channel, or conductor, and each of them requires pressure to force it onward. Water, however, being a tangible substance, requires a hollow conductor; while electricity, being intangible, will flow through a solid conductor. The iron of the water-pipe and the insulation of the electric wire serve the same purpose—namely, that of serving to prevent escape by reason of the pressure exerted.

There is another term which should be mentioned in connection with resistance, as they are closely related, and that isopposition. There is no general electrical term of this name, but, as it will be most easily understoodfrom the meaning of the word itself, we have used it.

Let us give an example of what opposition would mean if applied to water. Probably every one knows that a water-wheel is a wheel having large blades, or "paddles," around its circumference.

When the water, in trying to force its passage, rushes against one of these paddles it meets with its opposition, but overcomes it by pushing the paddle away. This brings around more opposition in the shape of another paddle, which the water also pushes away. And so this goes on, the water overcoming this opposition and turning the wheel around, by which means we can get water to do useful work for us.

You must remember, however, that it is only by putting opposition in the path of a pressure and quantity of water that we can get this work.

The same principle holds good in electricity. We make electricity in different ways, and in order to obtain useful work we put in its path the instruments, lamps, or machines which offer the proper amount of resistance, or opposition, to its passage, and thus obtain from this wonderful agent the work we desire to have done.

You have learned that three important measurements in electricity are as follows:

Thevoltis the practical unit of measurement ofpressure;

Theampèreis the practical unit of measurement of therate of flow; and

Theohmis the practical unit of measurement ofresistance.

IIIMAGNETISMNow we will try to explain to you something about magnets and magnetism. There are very few boys who have not seen and played with the ordinary magnets, shaped like a horseshoe, which are sold in all toy-stores as well as by those who sell electrical goods.Well, you know that these magnets will attract and hold fast anything that is made of iron or steel, but they have no effect on brass, copper, zinc, gold, or silver, yet there is nothing that you can see which should cause any such effect. You will notice, then, that magnetism is like electricity; we cannot see it, but we can tell that it exists, because it produces certain effects. And here is another curious thing—magnetism produces electricity, and electricity produces magnetism. This seems to be a very convenient sort of a family affair, and it is owing to this close relationthat we are able to obtain so many wonderful things by the use of electricity.We shall now show you how electricity produces magnetism, and, when we come to the subject of electric lighting we will explain how magnetism produces electricity.Fig. 3The easiest way to show how electricity makes magnetism is to find out how magnets are made. Suppose we wanted to make a horseshoe magnet, just mentioned above; we would take a piece ofsteeland wind around it some fine copper wire, commencing on one leg of the horseshoe and winding around until we came to the end of the other leg. Then we should have two ends of wire left, as shown in the sketch. (Fig. 3.)We connect these two ends with an electric battery, giving, say, two volts, and then the ampères of current of electricity will travel through the wire, and in doing so has such an influence on the steel that it is converted into a magnet, such as you have played with. The current is "broken"—that is tosay, it is shut off several times in making a magnet of this kind, and then the wire is taken away from the battery and is unwound from the steel horseshoe, leaving it free from wire, just as you have seen it. This horseshoe is now apermanent magnet—that is, it willalwaysattract and hold pieces of iron and steel.Now, if you were to do the same thing with a horseshoe made of soft iron instead of steel it would not be a magnet after you stopped the current of electricity from going through the wires, although the piece ofironwould be a stronger magnet while the electricity was going through the wire around it.The steel magnet is called a permanent magnet, and its ends, or "poles," are named North and South. There is usually a loose piece of steel or iron, called an "armature," put across the ends, which has the peculiar property of keeping the magnetism from becoming weaker, and thereby retaining the strength of the magnet. The strongest part of the magnet is at the poles, while, at the point marked + (which is called the neutral point) there is scarcely any magnetism.It will be well to remember the object of thearmatureas we shall meet it again in describing dynamo machines.The magnets made of iron are called electromagnets because they exhibit magnetism only when the ampères of current of electricity are flowing around them. They also have two poles, north and south, as have permanent magnets. Electromagnets are used in nearly all electrical instruments, not only because they are stronger than permanent magnets, but because they can be made to act instantly by passing a current of electricity through them at the most convenient moment, as you will see when we explain some of the electrical instruments which are used to produce certain effects. (Fig. 4.)Fig. 4Of course there are a great many different shapes in which magnets are made. The simplest is thebar magnet, which is simply a flat or round piece of iron or steel. Suppose you made a magnet of a flat piece of steel and put on top of it a sheet of paper, and then threw on the paper some iron filings, youwould see them arrange themselves as is shown in the following sketch. (Fig. 5.)The filings would always arrange themselves in this shape, no matter how large or small the magnets were. And, if you were to cut it into two or half a dozen pieces, each piece would have the same effect. This shows you that each piece would itself become a magnet and would have its poles exactly as the large one had.Fig. 5Now, we have another curious thing to tell you about magnets. If you present the north pole of a magnet to the south pole of another magnet, they will attract and hold fast to each other, but if you present a south pole to another south pole, or a north pole to a north pole, they will repel each other, and there will be no attraction. You can perform some interesting experiments by reason of this fact. We will give you one of them.Take, say, a dozen needles and draw them several times in the same direction across the ends of a magnet so that they become magnetized.Now stick each needle half-way through a piece of cork, and put the corks, with the needles sticking through them, into a bowl of water. Then take a bar magnet and bring it gradually toward the middle of the bowl and you will see the corks advance or back away from the magnet. If the ends of the needles sticking up out of the water are south poles and the end of the magnet you present is a north pole, the needles will come to the center; but will go to the side of the bowl if you present the south pole. You can vary this pretty experiment by turning up the other ends of part of the needles.You will remember that when we explained what "resistance" meant, we told you that electricity would always take the easiest path, and while part of it will flow in a small wire, the largest portion will take an easier path if it can get to something larger that is a metallic substance. Electricity will only flow easily through anything that is made of metal. You will also remember that you learned that when electricity took a short cut to get away from its proper path it was called ashort circuit.All this must be taken into consideration when magnets are being made. In the firstplace, the wire we wind around steel or iron to make magnets must always be covered with an insulator of electricity. Magnet wire is usually covered with cotton or silk. If it were left bare, each turn of the wire would touch the next turn, and so we should make such an easy path for the electricity that it would all go back to the battery by a short circuit, and then we would get no magnetic effect in the steel or iron.The only way we can get electricity to do useful work for us is to put some resistance or opposition in its way.So you see that if we make it travel through the wire around the iron or steel, there is just enough resistance or opposition in its way to give it work to get through the wire, and this work produces the peculiar effect of making the iron or steel magnetic.The covering on the wire, as you will remember, is called "insulation."

MAGNETISM

Now we will try to explain to you something about magnets and magnetism. There are very few boys who have not seen and played with the ordinary magnets, shaped like a horseshoe, which are sold in all toy-stores as well as by those who sell electrical goods.

Well, you know that these magnets will attract and hold fast anything that is made of iron or steel, but they have no effect on brass, copper, zinc, gold, or silver, yet there is nothing that you can see which should cause any such effect. You will notice, then, that magnetism is like electricity; we cannot see it, but we can tell that it exists, because it produces certain effects. And here is another curious thing—magnetism produces electricity, and electricity produces magnetism. This seems to be a very convenient sort of a family affair, and it is owing to this close relationthat we are able to obtain so many wonderful things by the use of electricity.

We shall now show you how electricity produces magnetism, and, when we come to the subject of electric lighting we will explain how magnetism produces electricity.

Fig. 3

The easiest way to show how electricity makes magnetism is to find out how magnets are made. Suppose we wanted to make a horseshoe magnet, just mentioned above; we would take a piece ofsteeland wind around it some fine copper wire, commencing on one leg of the horseshoe and winding around until we came to the end of the other leg. Then we should have two ends of wire left, as shown in the sketch. (Fig. 3.)

We connect these two ends with an electric battery, giving, say, two volts, and then the ampères of current of electricity will travel through the wire, and in doing so has such an influence on the steel that it is converted into a magnet, such as you have played with. The current is "broken"—that is tosay, it is shut off several times in making a magnet of this kind, and then the wire is taken away from the battery and is unwound from the steel horseshoe, leaving it free from wire, just as you have seen it. This horseshoe is now apermanent magnet—that is, it willalwaysattract and hold pieces of iron and steel.

Now, if you were to do the same thing with a horseshoe made of soft iron instead of steel it would not be a magnet after you stopped the current of electricity from going through the wires, although the piece ofironwould be a stronger magnet while the electricity was going through the wire around it.

The steel magnet is called a permanent magnet, and its ends, or "poles," are named North and South. There is usually a loose piece of steel or iron, called an "armature," put across the ends, which has the peculiar property of keeping the magnetism from becoming weaker, and thereby retaining the strength of the magnet. The strongest part of the magnet is at the poles, while, at the point marked + (which is called the neutral point) there is scarcely any magnetism.

It will be well to remember the object of thearmatureas we shall meet it again in describing dynamo machines.

The magnets made of iron are called electromagnets because they exhibit magnetism only when the ampères of current of electricity are flowing around them. They also have two poles, north and south, as have permanent magnets. Electromagnets are used in nearly all electrical instruments, not only because they are stronger than permanent magnets, but because they can be made to act instantly by passing a current of electricity through them at the most convenient moment, as you will see when we explain some of the electrical instruments which are used to produce certain effects. (Fig. 4.)

Fig. 4

Of course there are a great many different shapes in which magnets are made. The simplest is thebar magnet, which is simply a flat or round piece of iron or steel. Suppose you made a magnet of a flat piece of steel and put on top of it a sheet of paper, and then threw on the paper some iron filings, youwould see them arrange themselves as is shown in the following sketch. (Fig. 5.)

The filings would always arrange themselves in this shape, no matter how large or small the magnets were. And, if you were to cut it into two or half a dozen pieces, each piece would have the same effect. This shows you that each piece would itself become a magnet and would have its poles exactly as the large one had.

Fig. 5

Now, we have another curious thing to tell you about magnets. If you present the north pole of a magnet to the south pole of another magnet, they will attract and hold fast to each other, but if you present a south pole to another south pole, or a north pole to a north pole, they will repel each other, and there will be no attraction. You can perform some interesting experiments by reason of this fact. We will give you one of them.

Take, say, a dozen needles and draw them several times in the same direction across the ends of a magnet so that they become magnetized.Now stick each needle half-way through a piece of cork, and put the corks, with the needles sticking through them, into a bowl of water. Then take a bar magnet and bring it gradually toward the middle of the bowl and you will see the corks advance or back away from the magnet. If the ends of the needles sticking up out of the water are south poles and the end of the magnet you present is a north pole, the needles will come to the center; but will go to the side of the bowl if you present the south pole. You can vary this pretty experiment by turning up the other ends of part of the needles.

You will remember that when we explained what "resistance" meant, we told you that electricity would always take the easiest path, and while part of it will flow in a small wire, the largest portion will take an easier path if it can get to something larger that is a metallic substance. Electricity will only flow easily through anything that is made of metal. You will also remember that you learned that when electricity took a short cut to get away from its proper path it was called ashort circuit.

All this must be taken into consideration when magnets are being made. In the firstplace, the wire we wind around steel or iron to make magnets must always be covered with an insulator of electricity. Magnet wire is usually covered with cotton or silk. If it were left bare, each turn of the wire would touch the next turn, and so we should make such an easy path for the electricity that it would all go back to the battery by a short circuit, and then we would get no magnetic effect in the steel or iron.The only way we can get electricity to do useful work for us is to put some resistance or opposition in its way.So you see that if we make it travel through the wire around the iron or steel, there is just enough resistance or opposition in its way to give it work to get through the wire, and this work produces the peculiar effect of making the iron or steel magnetic.

The covering on the wire, as you will remember, is called "insulation."

IVTHE TELEGRAPHEvery one knows how very convenient the telegraph is, but there are not many who think how wonderful it is that we can send a message in a few seconds of time to a distant place, even though it were thousands of miles away. And yet, though the present system of telegraphing is a wonderful one, the method of sending a telegram is simple enough. The apparatus that is used in sending a telegram is as follows:The Battery.The Wire.The Telegraph Key.The Sounder.The different kinds of electric batteries will be mentioned afterward, so we will not stop now to describe them, but simply state that a battery is used to produce the necessary electricity. As you all know what wireis, there is no necessity of describing it further.The telegraph key is shown in the sketch below. (Fig. 6.)Fig. 6This instrument is usually made of brass, except that upon the handle there is the little knob which is of hard rubber. The handle, or lever, moves down when this knob is pressed, and a little spring beneath pushes it up again when let go. You will see a second smaller knob, the use of which we will explain later.The sounder is shown on the following page. (Fig. 7.)The part consisting of the two black pillars is an electromagnet, and across the top of these pillars is a piece of iron called the "armature," which is held up by a spring.Fig. 7Now let us see how the battery and wire are placed in connection with these instruments. You have seen that we usually have two wires for the electricity to travel in, one wire for it to leave the battery, and the other to return on. But you will easily see that if two wires had to be used in telegraphing it would be a very expensive matter, especially when they had to be carried thousands of miles. So, instead of using a second wire, we use the earth to carry back the electricity to the battery, because the earth is a better conductor even than wire. Although a quantity of ground equal in size to the wire would offer thousands of times greater resistance than the wire, yet, owing to the great bodyof our earth, its total resistance is even less than any telegraph wire used.When two electric wires are run from a battery and connected together through some instrument, this is called a "circuit," because the electricity has a path in which it can travel back to the battery. This would be a "metallic" circuit;but when one wire onlyis used, and the other side of the battery is connected with the earth, it is called a "ground" or "earth" circuit, because the electricity returns through the earth.Fig. 8If you look at this sketch (Fig. 8) you will see how the telegraph instruments are connected and will then be able to understand how a message can be sent.Here we have two sets of telegraph apparatus,one of which, let us say, is in New York and the other in Philadelphia.You will see that one wire from the battery is connected with the earth, and the other wire with the sounder. Another wire goes from the sounder to one leg of the key so as to make the brass base of the key part of the circuit. The other leg of the key is "insulated" from the brass base by being separated therefrom with some substance which will not carry electricity, such, for instance, as hard rubber.We will suppose that there is already a wire strung up on poles between New York and Philadelphia, and that the key, sounder, and battery in the latter city are connected in the same way as those in New York.Now, to enable us to send a message from one city to the other we must connect the ends of the wires to the instruments in each city; so we connect one end to the insulated leg of the key in New York, and the other end to the insulated leg of the key in Philadelphia.Everything is now completed, and, as soon as we find out what is the use of that part of the key that has a little round, black handle, we shall be ready to start. This is called the "switch."If you will look once more at the picture of the key you will see under the long handle (or lever) a little point which the lever will touch when it is pressed down. Now this little point is part of that insulated leg, and, therefore, this point is also insulated from the base. If a current of electricity were sent along the wire it could not get any farther than this point unless we put in some arrangement to complete the path, or circuit, for it to travel in. We therefore put in the switch.One end of the switch (which is made of brass with a rubber handle) is fastened on the base of the key, so that it may be moved to the right or left. The other end, when the switch is moved to the left (or "closed"), touches a piece of brass fastened to the little point we have mentioned, and so makes a free path for the electricity to go through the base of the key and through the wire to the sounder, and from there to the battery, and so back to the earth. This switch must be opened before the sounder near it will respond to its neighboring key.Now we are ready to send a message. Suppose we want to send a telegram from New York to Philadelphia. The operator in New York opens his switch and pressesdown his key several times. The switch on the Philadelphia key being closed, the electricity goes through to the sounder, and, this being made an electromagnet by the current passing through the wire, the iron armature is attracted by the magnetism and drawn down to the magnet with a snap. It will stay there as long as the New York operator keeps his lever pressed down, but, when he allows it to spring up, there is no current passing through the Philadelphia sounder and there is no magnetism, consequently the armature springs up again with a click.As often as the operator presses down his key lever and lets it spring up again, the same action takes place in the sounder, and it makes that click, click, which you have heard if you have ever seen telegraph instruments in operation.Let us continue, however, to send our message. The New York operator, having pressed down his key several times to signal the Philadelphia operator, closes his switch to receive the answer from Philadelphia. The operator in the latter city then opens his switch and presses down his key several times, which makes the New York sounder click, in the same way, to let the operator there know that he is ready to receive themessage. He then closes his switch and receives the telegram which the New York operator sends after openinghiskey.Telegraphic messages are sent and received in this way and are read by the sound of the clicks.These sounds may be represented on paper by dots, dashes, and spaces. For instance, if you press down the key and let it spring back quickly, that would represent a dot. If you press down the key and hold it a little longer before letting it spring up again, it would represent a dash. A space would be represented by waiting a little while before pressing down the key again.We show you below the alphabet in these dots, dashes, and spaces, and these are the ones now used in sending all telegraphic messages.Thus, you see, if you were telegraphing the word "and" you would press down your key and let it return quickly, then press down again and return after a longer pause, which would give the letter A; then slowly and quickly, which would be N; then slowly and twice quickly, which would be D.Any persevering boy can learn to operate a telegraph instrument by a little study and regular practice; and, as complete learner's sets can be purchased very cheaply, this affords a pleasant and useful recreation for boys.There are many cases where two boys living near each other have a set of telegraph instruments in their homes and run a wire from one house to the other, thus affording many hours of pleasant and profitable amusement.In giving the above explanation of telegraphing we have described only the simple and elementary form. In large telegraph lines, such as those of the Western Union, there are many more additional instruments used, which are very complicated and difficult to understand; such, for instance, as the quadruplex, by which four distinct messages can be sent over the same wire at the same time. We have, therefore, describedonly the simplest form in order to give the general idea of the working of the telegraph by electromagnetism, which is the principle of all telegraphing.When you study electricity more deeply you will find this subject and the many different instruments very interesting and wonderful.

THE TELEGRAPH

Every one knows how very convenient the telegraph is, but there are not many who think how wonderful it is that we can send a message in a few seconds of time to a distant place, even though it were thousands of miles away. And yet, though the present system of telegraphing is a wonderful one, the method of sending a telegram is simple enough. The apparatus that is used in sending a telegram is as follows:

The Battery.The Wire.The Telegraph Key.The Sounder.

The different kinds of electric batteries will be mentioned afterward, so we will not stop now to describe them, but simply state that a battery is used to produce the necessary electricity. As you all know what wireis, there is no necessity of describing it further.

The telegraph key is shown in the sketch below. (Fig. 6.)

Fig. 6

This instrument is usually made of brass, except that upon the handle there is the little knob which is of hard rubber. The handle, or lever, moves down when this knob is pressed, and a little spring beneath pushes it up again when let go. You will see a second smaller knob, the use of which we will explain later.

The sounder is shown on the following page. (Fig. 7.)

The part consisting of the two black pillars is an electromagnet, and across the top of these pillars is a piece of iron called the "armature," which is held up by a spring.

Fig. 7

Now let us see how the battery and wire are placed in connection with these instruments. You have seen that we usually have two wires for the electricity to travel in, one wire for it to leave the battery, and the other to return on. But you will easily see that if two wires had to be used in telegraphing it would be a very expensive matter, especially when they had to be carried thousands of miles. So, instead of using a second wire, we use the earth to carry back the electricity to the battery, because the earth is a better conductor even than wire. Although a quantity of ground equal in size to the wire would offer thousands of times greater resistance than the wire, yet, owing to the great bodyof our earth, its total resistance is even less than any telegraph wire used.

When two electric wires are run from a battery and connected together through some instrument, this is called a "circuit," because the electricity has a path in which it can travel back to the battery. This would be a "metallic" circuit;but when one wire onlyis used, and the other side of the battery is connected with the earth, it is called a "ground" or "earth" circuit, because the electricity returns through the earth.

Fig. 8

If you look at this sketch (Fig. 8) you will see how the telegraph instruments are connected and will then be able to understand how a message can be sent.

Here we have two sets of telegraph apparatus,one of which, let us say, is in New York and the other in Philadelphia.

You will see that one wire from the battery is connected with the earth, and the other wire with the sounder. Another wire goes from the sounder to one leg of the key so as to make the brass base of the key part of the circuit. The other leg of the key is "insulated" from the brass base by being separated therefrom with some substance which will not carry electricity, such, for instance, as hard rubber.

We will suppose that there is already a wire strung up on poles between New York and Philadelphia, and that the key, sounder, and battery in the latter city are connected in the same way as those in New York.

Now, to enable us to send a message from one city to the other we must connect the ends of the wires to the instruments in each city; so we connect one end to the insulated leg of the key in New York, and the other end to the insulated leg of the key in Philadelphia.

Everything is now completed, and, as soon as we find out what is the use of that part of the key that has a little round, black handle, we shall be ready to start. This is called the "switch."

If you will look once more at the picture of the key you will see under the long handle (or lever) a little point which the lever will touch when it is pressed down. Now this little point is part of that insulated leg, and, therefore, this point is also insulated from the base. If a current of electricity were sent along the wire it could not get any farther than this point unless we put in some arrangement to complete the path, or circuit, for it to travel in. We therefore put in the switch.

One end of the switch (which is made of brass with a rubber handle) is fastened on the base of the key, so that it may be moved to the right or left. The other end, when the switch is moved to the left (or "closed"), touches a piece of brass fastened to the little point we have mentioned, and so makes a free path for the electricity to go through the base of the key and through the wire to the sounder, and from there to the battery, and so back to the earth. This switch must be opened before the sounder near it will respond to its neighboring key.

Now we are ready to send a message. Suppose we want to send a telegram from New York to Philadelphia. The operator in New York opens his switch and pressesdown his key several times. The switch on the Philadelphia key being closed, the electricity goes through to the sounder, and, this being made an electromagnet by the current passing through the wire, the iron armature is attracted by the magnetism and drawn down to the magnet with a snap. It will stay there as long as the New York operator keeps his lever pressed down, but, when he allows it to spring up, there is no current passing through the Philadelphia sounder and there is no magnetism, consequently the armature springs up again with a click.

As often as the operator presses down his key lever and lets it spring up again, the same action takes place in the sounder, and it makes that click, click, which you have heard if you have ever seen telegraph instruments in operation.

Let us continue, however, to send our message. The New York operator, having pressed down his key several times to signal the Philadelphia operator, closes his switch to receive the answer from Philadelphia. The operator in the latter city then opens his switch and presses down his key several times, which makes the New York sounder click, in the same way, to let the operator there know that he is ready to receive themessage. He then closes his switch and receives the telegram which the New York operator sends after openinghiskey.

Telegraphic messages are sent and received in this way and are read by the sound of the clicks.

These sounds may be represented on paper by dots, dashes, and spaces. For instance, if you press down the key and let it spring back quickly, that would represent a dot. If you press down the key and hold it a little longer before letting it spring up again, it would represent a dash. A space would be represented by waiting a little while before pressing down the key again.

We show you below the alphabet in these dots, dashes, and spaces, and these are the ones now used in sending all telegraphic messages.

Thus, you see, if you were telegraphing the word "and" you would press down your key and let it return quickly, then press down again and return after a longer pause, which would give the letter A; then slowly and quickly, which would be N; then slowly and twice quickly, which would be D.

Any persevering boy can learn to operate a telegraph instrument by a little study and regular practice; and, as complete learner's sets can be purchased very cheaply, this affords a pleasant and useful recreation for boys.

There are many cases where two boys living near each other have a set of telegraph instruments in their homes and run a wire from one house to the other, thus affording many hours of pleasant and profitable amusement.

In giving the above explanation of telegraphing we have described only the simple and elementary form. In large telegraph lines, such as those of the Western Union, there are many more additional instruments used, which are very complicated and difficult to understand; such, for instance, as the quadruplex, by which four distinct messages can be sent over the same wire at the same time. We have, therefore, describedonly the simplest form in order to give the general idea of the working of the telegraph by electromagnetism, which is the principle of all telegraphing.

When you study electricity more deeply you will find this subject and the many different instruments very interesting and wonderful.

VWIRELESS TELEGRAPHYIf it has seemed extraordinary to you that only one wire should be necessary for sending a message by the electric telegraph, and that our earth can be used instead of a second wire, how much more wonderful it is to realize that in these days we can exchange telegraphic messages with different points without any connecting wires at all between them, even though the places be many hundred miles apart. Thus, two ships on the ocean, entirely out of sight of each other, may intercommunicate, or may telegraph to or receive despatches from a far-distant shore; indeed, telegraphy without wires has been accomplished across the Atlantic Ocean. In the language of the day, this is called "wireless telegraphy," although it is more correct to think of it as aerial, or space, telegraphy. As you will naturally want to know how thisis effected, we will try to explain the main principles in a simple manner.If you drop a stone into a quiet pond, you will see the water form into ring-like waves, or ripples, which travel on and on until they die away in the far distance. These waves are caused, as we have seen, by a disturbance of the body of water.Probably you have already learned in school that all known space is said to be filled with a medium called "ether," and that this medium is so exceedingly thin that it penetrates, or permeates, everything, so that it exists in the densest bodies as well as in free space. For the sake of obtaining a clear idea of this theory we may imagine that the ether envelops and permeates every thing in the entire universe. Hence we can easily realize that, although we cannot see or feel the ether, any disturbance of it will set it in wavelike motion.Modern science accounts for light, radiant heat, and electrical phenomena by reason of wavelike disturbances, vibrations, or pulsations of this ether. Thus, if you should strike a light, the ether would be disturbed, causing waves to form, which, like the waves in the water, would travel in every direction. When these waves reached the eyes of anotherperson within seeing distance, that person's eyes would be so acted upon by the waves that he would see the light which you had made, and would see it instantly, for light waves travel about 186,000 miles per second.So, if you create an electrical disturbance, the same kind of an effect will be produced; that is to say, waves in the ether will be created, or propagated, and will travel on and on in every direction. Now, if some form of electrical appliance can be made that will be of the right kind to respond to them (as the eye responds to light rays), these electric waves can be made practically useful for transmitting messages through space. This is just what has been done, and we will now give you a brief general description of one kind of apparatus used.For "sending," or "transmitting," as it is usually termed, there is used an induction-coil, having rather large brass balls on the secondary terminals; suitable batteries, a condenser, a Morse telegraph key, and an "aerial," or wire which is carried away up into the air vertically, and is made fast to a pole or special tower. When these are connected properly, the closing of the circuit with the key will cause sparks to jumpbetween the brass balls. This electrical discharge, or oscillation, is carried by the aerial into the upper air and causes intense pulsations in the ether, which set up waves as already mentioned. If the circuit is opened again the disturbance ceases. So, by alternately closing and opening the circuit, the Morse characters can be imitated.But how can these signals be received by the man for whom they are intended, who may be a hundred miles or more away? He has a "receiving" set, consisting of a sensitive relay, batteries, resistance-coils, a Morse register, an aerial, and a special device called a "coherer." This is the important part of the whole set, because it is sensitive to the electrical waves. It consists of a little glass tube about as large around as an ordinary lead-pencil, and perhaps two inches long. In the tube are two metallic plugs, each having a wire attached so that one wire projects from each end of the tube. The plugs are separated inside the tube by a very small space, and in this space are some metal filings. One wire from the coherer is connected to the aerial and the other to the ground. When there are no electrical ether waves to influence them, these filings, being loosely separated, are at rest and offer highresistance; but when the ether is disturbed by electrical vibrations and the waves arrive at the coherer (through the aerial), these filings are drawn together, or cohere. This lowers their resistance and they become a better conductor. Now, the coherer wires are also connected through a battery to the relay, which in turn is connected through another battery to a Morse register. Therefore, when the filings become a conductor, the current flows through them and the circuit to the relay is closed. That attracts an armature which closes the circuit of the Morse register and thus marks the electrical impulse on a strip of paper tape. In the mean time, a restoring device, called a "decoherer," operated also by the relay circuit, has tapped upon the coherer, thus shaking the filings loose again, so that they are ready to cohere again and register another impulse, or character. Thus, by pressing the key at the transmitting end for long or short periods, to represent Morse characters, long and short waves are propagated in the ether and are received and recorded at the receiving end through the coherer and other parts of the receiving set. In this way telegraphic messages are sent and received through space,between points separated by hundreds or thousands of miles.We have tried to describe to you the general principles underlying the art of wireless telegraphy as plainly as possible, using for illustration the simplest kind of apparatus employed for the practical sending and receiving of messages. At the present day there are several systems in actual practice, and with the growth of the art there have been many elaborations of apparatus that have come into use. For instance, the coherer is not as much used as formerly. In its place there are employed several kinds of "wave-detectors" as they are now termed, and in many of the systems the electrical pulsations are generated by a dynamo-machine instead of batteries. Then, again, instead of the messages being recorded by a Morse register at the receiving end, the operator receives them by means of a telephone receiver, through which he hears the Morse characters and writes them down in words as he hears them. Generally the aerial, or "antennæ," as it is sometimes named, consists of several wires, sometimes a large number, carried to a considerable height.There are a great many other details which might be written to explain all the complicatedapparatus which is used in some of the systems, but it is not intended in this book to offer more than a general explanation of main principles. We must leave it to you to study the details elsewhere if you so desire after you have read these pages.

WIRELESS TELEGRAPHY

If it has seemed extraordinary to you that only one wire should be necessary for sending a message by the electric telegraph, and that our earth can be used instead of a second wire, how much more wonderful it is to realize that in these days we can exchange telegraphic messages with different points without any connecting wires at all between them, even though the places be many hundred miles apart. Thus, two ships on the ocean, entirely out of sight of each other, may intercommunicate, or may telegraph to or receive despatches from a far-distant shore; indeed, telegraphy without wires has been accomplished across the Atlantic Ocean. In the language of the day, this is called "wireless telegraphy," although it is more correct to think of it as aerial, or space, telegraphy. As you will naturally want to know how thisis effected, we will try to explain the main principles in a simple manner.

If you drop a stone into a quiet pond, you will see the water form into ring-like waves, or ripples, which travel on and on until they die away in the far distance. These waves are caused, as we have seen, by a disturbance of the body of water.

Probably you have already learned in school that all known space is said to be filled with a medium called "ether," and that this medium is so exceedingly thin that it penetrates, or permeates, everything, so that it exists in the densest bodies as well as in free space. For the sake of obtaining a clear idea of this theory we may imagine that the ether envelops and permeates every thing in the entire universe. Hence we can easily realize that, although we cannot see or feel the ether, any disturbance of it will set it in wavelike motion.

Modern science accounts for light, radiant heat, and electrical phenomena by reason of wavelike disturbances, vibrations, or pulsations of this ether. Thus, if you should strike a light, the ether would be disturbed, causing waves to form, which, like the waves in the water, would travel in every direction. When these waves reached the eyes of anotherperson within seeing distance, that person's eyes would be so acted upon by the waves that he would see the light which you had made, and would see it instantly, for light waves travel about 186,000 miles per second.

So, if you create an electrical disturbance, the same kind of an effect will be produced; that is to say, waves in the ether will be created, or propagated, and will travel on and on in every direction. Now, if some form of electrical appliance can be made that will be of the right kind to respond to them (as the eye responds to light rays), these electric waves can be made practically useful for transmitting messages through space. This is just what has been done, and we will now give you a brief general description of one kind of apparatus used.

For "sending," or "transmitting," as it is usually termed, there is used an induction-coil, having rather large brass balls on the secondary terminals; suitable batteries, a condenser, a Morse telegraph key, and an "aerial," or wire which is carried away up into the air vertically, and is made fast to a pole or special tower. When these are connected properly, the closing of the circuit with the key will cause sparks to jumpbetween the brass balls. This electrical discharge, or oscillation, is carried by the aerial into the upper air and causes intense pulsations in the ether, which set up waves as already mentioned. If the circuit is opened again the disturbance ceases. So, by alternately closing and opening the circuit, the Morse characters can be imitated.

But how can these signals be received by the man for whom they are intended, who may be a hundred miles or more away? He has a "receiving" set, consisting of a sensitive relay, batteries, resistance-coils, a Morse register, an aerial, and a special device called a "coherer." This is the important part of the whole set, because it is sensitive to the electrical waves. It consists of a little glass tube about as large around as an ordinary lead-pencil, and perhaps two inches long. In the tube are two metallic plugs, each having a wire attached so that one wire projects from each end of the tube. The plugs are separated inside the tube by a very small space, and in this space are some metal filings. One wire from the coherer is connected to the aerial and the other to the ground. When there are no electrical ether waves to influence them, these filings, being loosely separated, are at rest and offer highresistance; but when the ether is disturbed by electrical vibrations and the waves arrive at the coherer (through the aerial), these filings are drawn together, or cohere. This lowers their resistance and they become a better conductor. Now, the coherer wires are also connected through a battery to the relay, which in turn is connected through another battery to a Morse register. Therefore, when the filings become a conductor, the current flows through them and the circuit to the relay is closed. That attracts an armature which closes the circuit of the Morse register and thus marks the electrical impulse on a strip of paper tape. In the mean time, a restoring device, called a "decoherer," operated also by the relay circuit, has tapped upon the coherer, thus shaking the filings loose again, so that they are ready to cohere again and register another impulse, or character. Thus, by pressing the key at the transmitting end for long or short periods, to represent Morse characters, long and short waves are propagated in the ether and are received and recorded at the receiving end through the coherer and other parts of the receiving set. In this way telegraphic messages are sent and received through space,between points separated by hundreds or thousands of miles.

We have tried to describe to you the general principles underlying the art of wireless telegraphy as plainly as possible, using for illustration the simplest kind of apparatus employed for the practical sending and receiving of messages. At the present day there are several systems in actual practice, and with the growth of the art there have been many elaborations of apparatus that have come into use. For instance, the coherer is not as much used as formerly. In its place there are employed several kinds of "wave-detectors" as they are now termed, and in many of the systems the electrical pulsations are generated by a dynamo-machine instead of batteries. Then, again, instead of the messages being recorded by a Morse register at the receiving end, the operator receives them by means of a telephone receiver, through which he hears the Morse characters and writes them down in words as he hears them. Generally the aerial, or "antennæ," as it is sometimes named, consists of several wires, sometimes a large number, carried to a considerable height.

There are a great many other details which might be written to explain all the complicatedapparatus which is used in some of the systems, but it is not intended in this book to offer more than a general explanation of main principles. We must leave it to you to study the details elsewhere if you so desire after you have read these pages.

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