VIIELECTRIC HEATING

Fig. 32

11.The Relay(Fig. 32).—Telegraphing from 3,000 to 10,000 miles under the ocean is full of difficulties not now to be explained.

Fig. 33

Of course when we attempt to telegraph many miles upon land we find that the resistance of the wire cuts down the strength of the current so that it will not move the sounder. This, however, is readily obviated by the relay devised by Morse. It simply serves as an automatic key to close acircuit. A diagram will make this clear (Fig. 33). Suppose the line wire to be very long and on account of its resistance the current is too feeble to operate a sounder. It is likely to be about .025 ampere where the local sounder may require .25 ampere or ten times as much. It is easily possible to wind a magnet (Fig. 33),R, such that .025 ampere will close the armaturea, so that it may complete a local circuit when it would not make noise enough for a sounder.Bmay represent a local battery of any desired strength which may operate the sounderSof that station as loudly as may be desired.

Fig. 34

12.Annunciator(Fig. 34).—We live in a fifth-floor apartment. When we pushthe button to call the elevator a No. 5 appears in the annunciator in the elevator car. This tells the elevator boy where the call comes from. Take out two or three screws and the annunciator opens, revealing a series of electro-magnets like the one shown inFig. 35. When an electric current passes around the coil it pulls back an iron catch and allows a number to drop so as to show through a small window. The elevator boy, having noted that the call is from the fifth floor, pushes up the number and the iron catch holds it until the coil is magnetized again by an electric current.

Fig. 35

Fig. 36

Fig. 37

The annunciator has a bell to call attention. A cable of six wires enters this annunciator (Fig. 36). One wire goes direct to the bell and the other five reach the bell through the separate coils of the electro-magnets which control the drops. But how areelectrical connections made between a moving elevator car and the push buttons on various floors? The diagram inFig. 37shows this in elevation.Brepresents a battery of several dry cells located in the basement. One wire from it runs direct to the push buttons 1, 2, 3, 4, 5, located upon the five floors of the house. The other wire from the battery, together with wires from each of the five push buttons, all run to a point,A, half-way up the elevator shaft. Here the six wires are gathered into a cable long enough to reach either to the top or the bottom of the elevator shaft. The other end of this cable enters the elevator car and runs to the annunciator. The wire from the battery goes direct to the bell. The wires from the various push buttons go through correspondingly numbered electro-magnets to the bell. When, therefore, we pushed the button on the fifth floor, we closed the gap in the electric circuit at that point. The currentcame up from the battery, passed through the button, went down the cable to the car, went through electro-magnet No. 5, went through the bell, and returned direct to the battery, thus completing the circuit. Annunciators are used about buildings to call other attendants, besides the elevator boy. They are likewise used in burglar alarms to inform the householder which door or window is being forced. They are used in the fire department to tell what part of the city the call came from.

Fig. 38

13.The Electric Bell and Buzzer(Fig. 38).—So common a thing as an electric bell really belongs tothe present generation. Bells were either novelties or toys when I was your age. They cost then many times what they do now and then were poorly made. Nobody dared to trust them for front-door bells. It was necessary to have a card permanently posted over the push button saying, "If the bell does not ring, knock." In those days batteries were troublesome to care for, houses were not wired when built, and no one had learned the art of concealing the wires neatly.

Fig. 39

The buzzer is simply a bell minus gong and hammer. Those shown inFig. 38ring well on a single dry cell. A cell costing twelve cents operated one for two years while it was used as a call bell from dining room to kitchen, the current required being .15 ampere.

Electric Bell

The connections are shown in the diagram (Fig. 39). Suppose the current to enter at the binding posta, pass around the magnetsband then to the postc. The armaturednormally rests against the postcand the current finds its way along this to the posteand thence back to the battery. But as soon as the current passes,bbecomesa magnet and pulls the armaturedaway from the postc, thus breaking the circuit, whenbceases to be a magnet and a spring pushes the armaturedback against the postcto repeat the operation. The armaturedcarries a hammer which strikes the gongf. If the wire, which is usually connected with the binding poste, is connected with the postc, the "clatter" bell is changed to a "single-stroke" bell, and if the gong and hammer are removed the "bell" is changed to a "buzzer."

Fig. 40

In the case of the buzzer, by changing the length of the armature or by weighting it, we may change the time of its vibrations and its tone. The connections between battery push button and bell form a complete circuit. InFig. 40Brepresents a battery, usually of dry cells,B'represents the bell, andPrepresents the push button. The electric circuit is "open," (that is, there is a break in the conductor) atPuntil some one "pushes the button," that is, simply pushes against a spring so as to cause a piece of metal to bridge the gap in the conductor. Then we say the circuit is "closed."

Fig. 41

Fig. 42

Push button devices and switches are innumerable. In every case they are simply devices for pushing one piece of metal against another and completing the circuit for an electric current. Every one should unscrew and examine a few of them, both for the pleasure of seeing how they work and to learn howto make them work when they sometimes fail. Not only in bells but in all other instruments where electro-magnets are used, the magnets are placed in pairs, fastened together upon an iron base. They are wound so that the free ends are made opposite poles by the electric current. Like a horseshoe magnet, they form one magnet. The two poles thus placed are mutually helpful and each is stronger than it would be if separated from the other.

Fig. 43

14.Electric Clocks, Self-winding Clocks, Programme Clocks.—A pretentious-looking thing which appeared like a dish pan with a glass bottom was opened by the boys and found to be the simplest of all clocks.It had an electro-magnet like that inFig. 44. A strip of iron acting as an armature across the free ends of this magnet, pushed like a finger against the cogs of a wheel. This wheel was on the axle of the minute hand and it had sixty cogs. The electric circuit was closed through the magnet for an instant each minute and the armature pushed the wheel ahead one cog. Thus it made one complete revolution in an hour. A train of four other cog-wheels caused the hour hand to trail after at one twelfth the speed of the minute hand. This machinery made simply a small handful in an eighteen-inch stamped-metal "dish-pan" costing fifteen dollars.

Fig. 44

A self-winding clock was opened and found to contain two dry battery cells, an electro-magnet which operated very much like that of a "clatter" bell, the hammer like a finger poking against the cogs of a wheel. Once an hour the long hand closed the circuit through the battery and the magnet and its armature swung back and forth long enough to give the cog wheel one complete revolution andwind a spring, which it carried upon its axle. This spring kept the clock running one hour, until the next winding.

The programme clocks which were examined were self-winding clocks, but were connected by wires to the master clock which corrected them each hour. Each time the long hand of the master clock came to twelve it closed an electric circuit through all the clocks in the system. In each clock the current passed around an electro-magnet and caused it to pull an armature against a metal stop and set each long hand exactly at twelve. This master clock is sometimes situated many miles away and may correct the time for a whole city. Thus a master clock at Washington, D. C., furnishes standard time to all parts of the United States. The masterclock which we examined also closed the circuit at proper intervals through a series of programme bells placed in the various class rooms, and these called and dismissed classes automatically.

Fig. 45

15.Watchman's Time Detector(Fig. 45).—This is a device to compel a watchman to make his appointed trips. Push buttons or switches are distributed about the building at various points, and it is made his duty to close the circuits at these points at stated times. When he does so, the fact is recorded by electro-magnets puncturing, or, in some way, marking a revolving time card in the clock.

Fig. 46

16.Circuit Breakers(Fig. 46).—Electro-magnets are used to open switches and thus protect dynamos and other machines against a larger electric current than they are able to carry. The switch is heldclosed by a spring which, by an adjusting device, may be tightened or loosened. A dynamo which we examined had its circuit breaker adjusted so that it would remain closed if any current under 1500 amperes passed, but if a greater current than that passed it would strengthen the magnet sufficiently to open the switch and thus break the circuit.

Fig. 47

17.Separating Iron from Ore.—In 1897 Edison first proposed to use an electro-magnet to separate iron from crushed earth.Fig. 47represents the process.Eis an electro-magnet.Sis the stream of crushed ore containing iron. Gravity would cause all the material to fall into binA, but the electro-magnetEpulls that portion of the material which is magnetic to one side so that it falls into the binB.

Fig. 48

18.Lifting Magnets.—Electro-magnets are made for use with hoisting apparatus to save the trouble of manipulating grappling hooks, etc. They maylift barrels and boxes of iron, the wood of the barrel or box being transparent, we say, to the magnetic influence. That is, the magnet will attract iron through the wood just as light will shine through glass. Such magnets are used to pick up from the bottom of the sea cases of hardware from wrecked ships. (See the accompanying illustration,Fig. 48.) In such cases the electric conductors which lead to and encircle the magnets must be well insulated from the water of the sea, otherwise the electric current would take the shorter path from one line wire through the sea water, which is a fairly good conductor, and back by the other line wire, rather than go the path of greater resistance around the magnet. Electro-magnets are coming into use in foundries, etc., for lifting heavy iron castings.

Fig. 49

19.Electro-Magnet on Starting Box.—As was explained underelectric motors, a starting box is simply a series of resistance coilsr,r,r,r,r, inFig. 49. When the motor is not in use the switchlrests upon the point 1 and no electric current passes.When the switch is moved to point 2, the current entering atapasses to the pivot of the switch and up the metal striplto the point 2, then around the series of coils,r,r,r,r,r, to the postband thence back to the generator. As the switch is moved to the right, the current passes through less and less of this resistance until, when it reaches point 7, all the coils of resistance are "cut out," that is, they are not in the path of the current. Now the motor has reached its full speed and is developing enough counter-electro-motive force to protect itself against too much current. Through a shunt, however, a portion of the current passes fromatobaround the electro-magnete, the two poles of which are presented to the metal stripl, which must be of iron. This magnet holds the switch over so long as the current is on, but when the current is cut off, by opening a switch in the line wire,eceases to be a magnet andlis carried back to point 1 by a spring. Thus an extra resistance must always be in circuit when the motor is first started. Those who start motors are expected to move the leverlof the starting box slowly from point to point, pausing a second or two on each to give the motor time to acquire proper speed for its protection.How too great a current would "burn out" a motor will be explained later.

The motor man handles a lever for starting his car, which works like that of the "starting box." His "starting box," however, is called a "controller." Although it accomplishes the same result as the starting box it has a wholly different and vastly more complex mechanism than that already described.

The elevator boy, who runs our electric elevator, handles a lever which also does the same thing through far different mechanism. Indeed, in his case electro-magnets are used to prevent him from cutting out resistance too fast if he should move his lever too quickly.

20.Starting Switches for Electric Elevators.—The motor man has to be instructed particularly how he should handle the lever of his controller, and he is trusted to follow his directions to some extent, however lacking in intelligence and integrity he may be. But the elevator boy receives scarcely any instructions about his machine, and, indeed, his machine has been constructed pretty nearly "foolproof." It will automatically correct his errors of management. If he throws the handle from one extreme to the other, all resistance cannot bethrown out instantly, but this is accomplished by a series of electro-magnets closing one switch after another and thus cutting out resistance gradually.

21.Arc Lamp Feed.—As will be explained later, an arc lamp must have its carbons touching one another when the current is first thrown on, and then the carbons must be drawn apart from a quarter to half an inch. The upper carbon is lifted away from the lower one by a portion of the current passing by means of a shunt around an electro-magnet.

Fig. 50

Fig. 51

Fig. 52

22.Volt meter.—The volt meter measures the pressure of an electric current. The volt meter which we examined looked outside like our ammeter, and when we removed the face it appeared inside like an ammeter. There was the steel magnet of horseshoe shape to furnish a field (Fig. 51), and there was an electro-magnet poised between its poles for an armature. The armature in the volt meter, however,had wound upon it finer wire and more of it than was the case in the ammeter. There was no shunt wire in the volt meter as there was in the ammeter. We connected in series a fluid cell (to be described later), the ammeter, and the volt meter (Fig. 52). The ammeter shunt was removed so that all the current went through its armature. The volt meter needle went to one which was two thirds of the scale (Fig. 53), and the ammeter needle indicated .016. That is, this particular cell can push sixteen thousandths of an ampere through the resistance of this volt meter, and .016 amperepassing through the armature of this volt meter will magnetize it sufficiently to move it against its spring, say sixty degrees.

Fig. 53

Fig. 54

We put into the circuit a lot more fine wire for resistance,R(Fig. 54), so that the volt meter needle went only half as far as before, that is to .5. The ammeter indicated only half as much as before, that is .008 ampere. We put in resistance enough to bring the volt meter needle down to .25 and the ammeter indicated one quarter of the original current. We put in less resistance, bringing the volt meter needle to .75, and the ammeter indicated three fourths of the original current. Evidently thevolt meter is merely an ammeter with a different scale marked upon its card. With a pen we marked upon the card of the volt meter a true ammeter scale (Fig. 55).

Fig. 55

Fig. 56

In order to understand the volt meter, let us turn our attention for a moment toFig. 56. I have arranged the water tankTat such a height above the faucetFthat when the faucet is opened one quart of water will flow in a minute. If I partially close the faucet, making the opening one half as large (that is, offering twice the resistance to the flow), half a quart will flow in a minute. If I make the resistance four times as great only one quarter of a quart will flow in a minute. It is evident that I could arrange a scale underneath the handle of the faucet to indicate the quantity of water flowing, just as the ammeter and volt meter indicate the quantity of electricity which flows. If now that much is understood, it will be easy to learn how the water faucet may be used to measure water pressure and the volt meter in like manner used to measure electric pressure.

Having set the faucet so that a quart will flow per minute, let us put on a longer tubep, and move the tank up to another shelf so that the distance from the water level in the tank to the faucet is twice as great as before. Under the increased pressure water runs through the faucet twice as fast and we now get two quarts per minute.

I purposely placed the tank out of sight behind a partition so that you might practise judging the water pressure by the flow at the faucet. We cannot very well talk about pressure in quarts. We might talk about it in pounds, but if we used this apparatus much we should probably get into the habit of talking about the pressure from one shelf, two shelves, three shelves, etc.

In order that the pressure might remain nearly constant during the experiment we would probably introduce resistance (that is, partially close the faucet) so that the water level should not fall much. We might, for example, set the faucet so that half a pint would flow in a minute when the tank was on the first shelf. Then a pint per minute would flow when the tank was on the second shelf and one and a half pints per minute when the tank was on the third shelf, etc. Thus we should infer the pressure by measuring the quantity.

One more illustration and the case will be clear. To save the trouble of measuring the quantity of water which flows through the faucet, suppose I introduce the device represented inFig. 57.Wis a small water wheel comparable to the armature of the volt meter. It carries a pointer which moves over a scale just as in the case of the volt meter.

Fig. 57

It has a spring coiled around its axle which tends to keep the pointer at0, as in the case of the volt meter. The tank is placed upon the first shelf, the faucet is fixed so that a small amount of water flows and the needle moves to a certain figure upon the scale. We will mark this point one and call it "first-shelf pressure." The tank is lifted to the second shelf and the index moves to another point, which we will mark two and call it "second-shelf pressure." The tank is lifted to the third shelf and the index moves to a third point, which we will mark three and call it "third-shelf pressure," etc.

Ordinarily we measure water pressure with an instrument which allows no water to run to waste,but in measuring electric pressure by the volt meter some current must pass through the instrument, just as in the case of our water-wheel illustration inFig. 57. We put in large resistance so as to make this current as small as possible, while we let enough pass to move the armature.

Fig. 58

Now let us return to the volt meter itself. By referring toFig. 55, we see that it requires .024 ampere to move the needle of the volt meter clear across the scale, and we have found that one fluid cell was able to send enough current through the resistance of the armature to move the needle two thirds of the way across the scale. At this point we findFig. 1, which might be read "one-cell pressure." We prefer to commemorate the name of one of the workers in the field of electricity and call this pressure a "volt" after Alessandro Volta (1745–1827), born at Como, Italy. It is the electric pressure which is produced by one fluid cell of a certain kind. We say, then, that one volt pushes through the resistance of this armature .016 ampere. Half a volt would push through the resistance of the armature half as much currentor .008 ampere. At this point we put .5. Thus each of the figures in the lower row (Fig. 55) shows what part of a volt is required to send enough current through this particular armature to move the needle to that point.

Fig. 59

We found out how much wire was wound upon the armature and put exactly the same amount in the outside resistance,R(Fig. 59). The needle now showed that one volt is able to push through twice the resistance of the armature only half as much current, and the needle stopped at .008 ampere. If this were to be the resistance in the volt meter circuit one volt should stand under .008 ampere and two under .016 and three under .024. It is evident then, that, if we know the internal resistance of a volt meter, we may make it capable of measuring greater electrical pressures by adding the proper amount of resistance. By putting atR, (Fig. 59) nine times the internal resistance of the instrument, thus multiplying the total resistance tenfold, the figures upon the scale of volts may be read as whole numbers from one to fifteen. In this case it will require fifteen cells to push the needle clear acrossthe scale and ten cells to push it two thirds of the way across. If now we add enough external resistance to multiply the resistance of the armature a hundred fold it will require 150 volts to push .024 of an ampere through the armature and pull its needle clear across the scale. In this case the figures upon the scale of volts are multiplied by one hundred and read from ten to one hundred and fifty. Such a scale would adapt this volt meter for use with our 110-volt lighting circuit. Volt meters are made with a series of such external resistances, called "multipliers," attached so that they may be easily thrown into the circuit.

It is evident that we need some term so that we may speak of quantities of resistance. This need has given rise to a unit of resistance called an ohm, after George Simon Ohm (1789–1854) born at Erlanger in Bavaria. Two inches of No. 36 German silver wire, such as is wound upon the armature of this volt meter, gives one ohm of resistance. There are 125 inches of this wire upon the armature. Its resistance is, therefore, 62.5 ohms, and we may, therefore, say that one volt of electric pressure can push through 62.5 ohms of resistance .016 of an ampere of current. Ohmdiscovered this relationship in 1827, and formulated it as follows:

volts/ohms = amperes (not, however, using these words).(1 volt)/(62.5 ohms) = .016 ampere.62.5)1.0000(.016625——37503750——

This is called Ohm's law, as every candidate for college admission will hear and hear again.

Fig. 60

Volt meters and armatures for the alternating current have electro-magnets for their fields as well as for their armatures. Such instruments are equally well adapted for either direct or alternating currents. For when the current reverses its direction it reverses in field and armature alike, and thus a repulsion between like poles is maintained. Such an instrument, however, cannot respond to as slight a current as those previously described, since they must consume some energy in both field and armature.

Fig. 61

23.Telephone Receiver(Fig. 61).—It requires astretch neither of the imagination nor of the truth to call a telephone receiver an electro-magnet, although perhaps it has never been called that before. We took it apart and found that it consisted of a steel-bar magnetm(Fig. 62), with a small spool of wirewaround one end of it. The ends of the wire on the spool run along inside the hard rubber shell to the two binding postsaandbat the other end. A disk of sheet ironSis held in the large end of the case very near to, but not quite touching, the end of the magnet. When an alternating current is sent through the wire upon the spool it causes rapid changes in the strength of the magnetic field, if not reversals of the poles of the field, and the iron disk is made to vibrate, keeping time with the alternations of the current.

Fig. 62

In this laboratory we have seen that our current has sixty alternations per second. When it is connected with the receiver the disk, therefore, makes sixty vibrations per second, and producesa tone which has very nearly the pitch of C two octaves below the middle C upon the piano.

Fig. 63

24.Spark Coil(Fig. 63).—The automobile spark coil which we have already used is an electro-magnet. The battery sends a current through wire coiled around an iron core. At one end of this iron core is an iron armature which is made to vibrate in precisely the same manner as the armature of an electric bell. This makes and breaks the current and causes rapid changes in the strength of the field. A rapidly changing magnetic field may be used to develop electricity in a conductor, as we have already seen in the case of the dynamo.

How it is used in the automobile spark coil will be shown later. It is sufficient now to mention it as a case of a magnetic field produced by an electric current passing through a wire coiled around an iron core, or, in short, an electro-magnet.

Induction coils, Ruhmkorff coils, and transformers, to be described later, are closely related to this. They all create magnetic fields in the same way and are all electro-magnets.

Fig. 64. Transformers

Itwas Washington's birthday. The schools were to have a holiday and the Science Club was to hold a special, open meeting at which I had been asked to present the subject of electricity in the household. I replied to the programme committee that that was too large a subject, but that I would talk upon electric heating. I warned them, however, that it would be a dry study, and not an entertainment. They replied that the father of his country had been born at a time of the year when the weather was unfavourable to outdoor sports, and that February usually found them acclimated to vigorous study. Neither they nor their friends objected to study if it seemed to have a motive.

I found an audience composed of old and young, men and women, girls and boys. Most of them had left school—many of them because their teachers thought they were incompetent to continue.

Fig. 65

Not far from here is "a wheel in the middle of a wheel ... as for their rings they are so high that they are dreadful ... and the spirit of the living creature is in the wheels." Those wheels are now sending the electric current to this room for our experiments. I propose to show that we convert electricity into heat by offering resistance to its flow. Experience teaches us that resistance to motion always producesheat. At Niagara Falls thousands of tons of water descend at the rate of one hundred and sixty feet in three seconds. When the water reaches the bottom of the falls, it is moving a little faster than a mile a minute. The resistance which this mass meets after its fall retards its motion and generates heat.

Hundreds of meteors fall into our atmosphere daily, travelling a thousand times as fast as the waters of Niagara Falls. The resistance to their motion, which our atmosphere offers, heats them white hot, melts them, vaporizes them, burns them up, so that very few of them reach the solid earth in a solid condition.

An iron spile driver, measuring two cubic feet, weighs about half a ton. When it falls sixteen feet upon the end of a spile it is moving at the rate of twenty miles an hour. The energy of this moving mass depends upon both its weight and its velocity, and when its motion is arrested by the spile that energy of motion is largely converted into heat energy, from which both the spile and the spile driver get hot.

A piece of iron may be made red hot by pounding it with a trip hammer.

Count Rumford found, in 1798, while boring cannon in the arsenal at Munich, that the resistance which the iron offered to the motion of the boring tool furnished heat enough to boil water.

Seven hundred and seventy-eight foot pounds of mechanical energy when converted into heat would raise one pound of water (one pint) one degree. This is called the British thermal unit. The spile driver, weighing 1000 pounds, falling 16 feet upon a spile, produces heat enough to raise 1 pint of water 20 degrees.

Fig. 66

Here are two binding posts,aandb, 8 feet apart (Fig. 66), connected by copper wires with the dynamo circuit. The volt meter indicates 112 volts of pressure. I will close the circuit by stretching betweenaandb8 feet of No. 24 iron wire. (This wire is about the thickness of a common pin.) The iron wire offers resistance to the flow of the electric current, thereby producing heat—heat enough as you see to make the wire white hot, indeed heat enough to raise it to something over two thousand degrees Fahr., for now you see it has melted.

We will put in a fresh piece of wire and connect also the ammeter in the circuit (Fig. 67). As I close the circuit the needle of the ammeter at first indicates 20 or 30 amperes, but in a second drops to8 amperes, and remains there a second until the wire melts and falls apart. One hundred and twelve volts of electric pressure are able to push 8 amperes of electricity through this wire when hot.

Fig. 67

(112 volts)/(14 ohms) = 8 amperes112 volts × 8 amperes = 896 watts746 watts = one horse-power

Hence it required about one and one fifth horse-power to melt the wire in a second, and the heat produced was a little less than one British thermal unit, a unit much used by engineers.

1 pound raised 1 foot = 1 foot pound550 foot pounds per second = 1 horse-power778 foot pounds (1.4 H.-P.) = 1 B. T. U. (British thermal unit) = heat required to raise 1 pound of water 1° Fahrenheit1 volt × 1 ampere = 1 watt746 watts = 1 horse-power

1 pound raised 1 foot = 1 foot pound

550 foot pounds per second = 1 horse-power

778 foot pounds (1.4 H.-P.) = 1 B. T. U. (British thermal unit) = heat required to raise 1 pound of water 1° Fahrenheit

1 volt × 1 ampere = 1 watt

746 watts = 1 horse-power

In order to hold back 112 volts of electric pressure so that not more than eight amperes of electricityshould pass, the iron wire must have offered about 14 ohms of resistance.

The behaviour of the ammeter needle showed that the wire offered very much less resistance when cold than when hot. Indeed eight feet of No. 24 iron wire offers about one and one third ohms resistance when cold, hence heat had increased its resistance to the passage of the electric current tenfold.

This piece of iron wire offered resistance to the flow of the electric current. It offered resistance to the motion of the dynamo. This offered resistance to the steam-engine which drives the dynamo. This caused the governor of the engine to open and pass more steam from the boiler. This reduced the pressure at the steam gauge. This caused the fireman to shovel more coal into the furnace. The heat of the burning coal melts the wire, but it does it only after several changes. First, it is converted into mechanical energy in the steam-engine with great loss—about nine tenths being lost. Second, it is converted into electrical energy by the dynamo, with some loss, and, third, it is conducted to the iron wire and converted back to heat with still further loss. It is evident that the most economical way to heat the wire would beto take it to the furnace. Yet all electric cooking is done by sending electric current through wires embedded in the walls of the cooking utensils, and it is the most wasteful method of using the energy stored in coal that has yet been devised.

Fig. 68

That merely connecting the binding postsaandb(Fig. 67) by a small piece of wire should throw a load upon the dynamo miles away; should offer resistance to its motion, and make it require 1.18 horse-power more of energy to keep up its speed of revolution, is, indeed, uncanny. I will attempt to make it seem more real. At one end of the lecture table I have a rotary pumpP(Fig. 68). The end of the rubber tubea, which leads to the pumpis lying upon the table outside of the tank of water,T. While things are in this condition I move the crank which operates the pump with perfect ease. Now while still turning the crank I pick up the tubeaand drop its free end into the water tank. I cannot now conceal the fact, even if I were disposed to do so, that I must work hard to keep the pump going. The pump itself tells you by its laboured sound that it is working hard, and the stream of water which issues from the pipebtells how much work I am performing. The pump is discharging five and a half pints of water per second, that is 5.5 pounds, and it raises this water 10 feet. Hence I am doing 55 foot pounds of work per second, which requires one tenth of a horse-power. Here is a lad who consents to try the experiment for us. He turns the crank easily while I am holding the tubeaout of the water, but when I lower it into the water he finds the resistance so great that, tug however much he may, he is unable to keep the pump going.

At the other end of the table I have a small hand dynamo,D(Fig. 68),Mis an ammeter,Vis a volt meter,Sis a switch. All the wires are good-sized copper, and offer little resistance, except that stretched between the binding postsaandb. Thisis a piece of fine German silver wire. While the switch is open I turn the crank of the dynamo with perfect ease. A small amount of current is going through the volt meter, but this is too slight to offer any perceptible resistance to the motion of the machine.

Notice that the volt meter needle moves according to the speed of revolution. If I turn the crank once a second the needle stands at 25 volts. The electric pressure increases or decreases according to whether I rotate the armature faster or slower. Now I will attempt to keep the machine revolving at a constant rate while I close the switchS, and surely you must see that I have hard work to do so. The wirea bhas now become red hot. The volt meter shows 25 volts of pressure, and the ammeter shows 3 amperes of current.

Twenty-five volts × 3 amperes = 75 watts, which require one tenth of a horse-power (746 watts = 1 horse-power). The lad now takes my place at turning the machine and finds it easy when the switch is open, but I actually overload him by merely closing the switch. Heating the wire red hot requires more energy than he is able to put forth.

I proposed to the president that my lecture close at this point, and that each one in the room have achance tofeelthe load which was thrown upon the dynamo each time it was required to heat the wire. I suggested that each person should get a realizing sense of this fact, first by doing the work himself, and second by going home and reflecting upon this hint. When the switch is closed three amperes of electricity pass around the circuit. This increases the magnetism in both the field and the armature of the dynamo, and it requires one tenth of a horse-power more to keep the armature moving within the field against this magnetic pull.

I further desired to announce that during this hour I had delivered to them the second key to the Electrical Show which I had promised a few days ago. The second key is:

Heat (and light) is produced by offering resistance to the flow of the electric current. The first key is the electro-magnet. These two unlock all the mysteries of the show.

The president closed the formal exercises with the facetious remark that I had warned them before the lecture that they must work, so now each would be expected to take a turn at the cranks of the pump and dynamo.

Theprogramme committee decided that each member of the Science Club should busy himself looking forapplications of electric heatingand should consult me freely about the matter. My telephone was kept busy, my laboratory was in great demand, and we were all getting a good deal more education than the school was giving us credit for.

The boys generally came to me in pairs, and each pair having worked up some illustration of heat produced by electricity reported it to the club. These were spread by the secretary in due form upon the minutes of the club and constituted "The Proceedings of the Science Club."

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