Fig. 84. Telephone Stations in CircuitFig. 84.Telephone Stations in Circuit
Now, supposing the switch (D'), in station 2, should be thrown over so it contacts with the wire (H'). It is obvious that the current will then flow from the battery (I') through wires (H', C') and line (A) to station 1; then through wire C, switch D, wire E to the bell F, to the ground through wire G. From wire G the current returns through the ground to station 2,p. 118where it flows up wire G' to the battery, thereby completing the circuit.
Fig. 85. Illustrating Light Contact PointsFig. 85.Illustrating Light Contact Points
The operator at station 2, having given the signal, again throws his switch (D') back to the position shown in Fig.84, and the operator at station 1 throws on his switch (D), so as to ring the bell in station 2, thereby answering the signal, which means that both switches are again to be thrown over so they contact with the battery wires (H and H'), respectively. When both are thus thrown over, the bells (G, G') are cut out of the circuit, and the batteries are both thrown in, so that the telephones are now ready for talking purposes.
Microphone.—Originally this form of telephone system was generally employed, but it was found that for long distances a more sensitive instrument was necessary.
Light Contact Points.—In 1877 Professor Hughes discovered, accidentally, that a light contact point in an electric circuit augmented the sound in a telephone circuit. If, for instance, ap. 119light pin, or a nail (A, Fig.85) should be used to connect the severed ends of a wire (B), the sounds in the telephone not only would be louder, but they would be more distinct, and the first instrument made practically, to demonstrate this, is shown in Fig.86.
Fig. 86. MicrophoneFig. 87. TransmitterFig. 86.MicrophoneFig. 87.Transmitter
How to Make a Microphone.—This instrument has simply a base (A) of wood, and near one end is a perpendicular sounding-board (B) of wood, to one side of which is attached, by wax or otherwise, a pair of carbon blocks (C, D). The lower carbon block (C) has a cup-shaped depression in its upper side, and the upper block has a similar depression in its lower side. A carbon pencil (E) is lightly held within these cups, so that the lightest contact of the upper end of the pencilp. 120with the carbon block, makes the instrument so sensitive that a fly, walking upon the sounding-board, may be distinctly heard through the telephone which is in the circuit.
Microphone the Father of the Transmitter.—This instrument has been greatly modified, and is now used as a transmitter, the latter thereby taking the place of the pin (A), shown in Fig.85.
Automatic Cut-outs for Telephones.—In the operation of the telephone, the great drawback originally was in inducing users of the lines to replace or adjust their instruments carefully. When switches were used, they would forget to throw them back, and all sorts of trouble resulted.
It was found necessary to provide an automatic means for throwing in and cutting out an instrument, this being done by hanging the telephone on the hook, so that the act merely of leaving the telephone made it necessary, in replacing the instrument, to cut out the apparatus.
Before describing the circuiting required for these improvements, we show, in Fig.87, a section of a transmitter.
A cup-shaped case (A) is provided, made of some insulating material, which has a diaphragm (B) secured at its open side. This diaphragm carries the carbon pencil (C) on one side and from the blocks which support the carbon pencilp. 121the wires run to binding posts on the case. Of course the carbon supporting posts must be insulated from each other, so the current will go through the carbon pencil (C).
Complete Circuiting with Transmitter.—In showing the circuiting (Fig.88) it will not be possible to illustrate the boxes, or casings, which receive the various instruments. For instance, the hook which carries the telephone or the receiver, is hinged within the transmitter box. The circuiting is all that it is intended to show.
Fig. 88. Complete Telephonic CircuitFig. 88.Complete Telephonic Circuit
The batteries of the two stations are connected up by a wire (A), unless a ground circuit is used. The other side of each battery has a wire connection (B, B') with one terminal of the transmitter, and the other terminal of the transmitter has a wire (C, C') which goes to the receiver. From the other terminal of the receiver is a wire (D, D') which leads to the upper stop contact (E, E') ofp. 122the telephone hook. A wire (F, F') from the lower stop contact (G, G') of the hook goes to one terminal of the bell, and from the other terminal of the bell is a wire (H, H') which makes connection with the line wire (A). In order to make a complete circuit between the two stations, a line wire (I) is run from the pivot of the hook in station 1 to the pivot of the hook in station 2.
In the diagram, it is assumed that the receivers are on the hooks, and that both hooks are, therefore, in circuit with the lower contacts (G, G'), so that the transmitter and receiver are both out of circuit with the batteries, and the bell in circuit; but the moment the receiver, for instance, in station 1 is taken off the hook, the latter springs up so that it contacts with the stop (E), thus establishing a circuit through the line wire (I) to the hook of station 2, and from the hook through line (F') to the bell. From the bell, the line (A) carries the current back to the battery of station (A), thence through the wire (B) to the transmitter wire (C) to receiver and wire (D) to the post (E), thereby completing the circuit.
When, at station 2, the receiver is taken off the hook, and the latter contacts with the post (E'), the transmitter and receiver of both stations are in circuit with each other, but both bells are cut out.
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Decomposing Liquids.—During the earlier experiments in the field of electricity, after the battery or cell was discovered, it was noted that when a current was formed in the cell, the electrolyte was charged and gases evolved from it. A similar action takes place when a current of electricity passes through a liquid, with the result that the liquid is decomposed—that is, the liquid is broken up into its original compounds. Thus, water is composed of two parts, by bulk, of hydrogen and of oxygen, so that if two electrodes are placed in water, and a current is sent through the electrodes in either direction, all the water will finally disappear in the form of hydrogen and oxygen gases.
Making Hydrogen and Oxygen.—During this electrical action, the hydrogen is set free at the negative pole and the oxygen at the positive pole. A simple apparatus, which any boy can make, to generate pure oxygen and pure hydrogen, is shown in Fig.89.
It is constructed of a glass or earthen jar (A), preferably square, to which is fitted a wooden topp. 124(B), this top being provided with a packing ring (C), so as to make it air-tight. Within is a vertical partition (D), the edges of which, below the cap, fit tightly against the inner walls of the jar. This partition extends down into the jar a sufficient distance so it will terminate below the water level. A pipe is fitted through the top on each side of the partition, and each pipe has a valve. An electrode, of any convenient metal, is secured at its upper end to the top of the cap, on each side of the partition. These electrodes extend down to the bottom of the jar, and an electric wire connects with each of them at the top.
Fig. 89. Device for Making Hydrogen and OxygenFig. 89.Device for Making Hydrogen and Oxygen
If a current of electricity is passed through the wires and the electrodes, in the direction shownp. 125by the darts, hydrogen will form at the negative pole, and oxygen at the positive pole. These gases will escape upwardly, so that they will be trapped in their respective compartments, and may be drawn off by means of the pipes.
Purifying Water.—Advantage is taken of this electrolytic action, to purify water. Oxygen is the most wonderful chemical in nature. It is called the acid-maker of the universe. The name is derived from two words,oxyandgen; one denoting oxydation, and the other that it generates. In other words, it is thegenerator of oxides. It is the element which, when united with any other element, produces an acid, an alkali or a neutral compound.
Rust.—For instance, iron is largely composed of ferric acid. When oxygen, in a free or gaseous state, comes into contact with iron, it produces ferrous oxide, which is recognized as rust.
Oxygen as a Purifier.—But oxygen is also a purifier. All low forms of animal life, like bacteria or germs in water, succumb to free oxygen. Byfree oxygenis meant oxygen in the form of gas.
Composition of Water.—Now, water, in which harmful germs live, is one-third oxygen. Nevertheless, the germs thrive in water, because the oxygen is in a compound state, and, therefore, notp. 126an active agent. But if oxygen, in the form of gas, can be forced through water, it will attack the germs, and destroy them.
Common Air Not a Good Purifier.—Water may be purified, to a certain extent, by forcing common air through it, and the foulest water, if run over rocks, will be purified, in a measure, because air is intermingled with it. But common air is composed of four-fifths nitrogen, and only one-fifth oxygen, and, as nitrogen is the staple article of food for bacteria, the purifying method by air is not effectual.
Pure Oxygen.—When, however, oxygen is generated from water, by means of electrolysis, it is pure; hence is more active and is not tainted by a life-giving substance for germs, such as nitrogen.
The mechanism usually employed for purifying water is shown in Fig.90.
A Water Purifier.—The case (A, Fig.90) may be made of metal or of an insulating material. If made of metal it must be insulated within with slate, glass, marble or hard rubber, as shown at B. The case is provided with exterior flanges (C, D), with upper and lower ends, and it is mounted upon a base plate (E) and affixed thereto by bolts. The upper end has a conically-formed cap (F) bolted to the flanges (C), and this has an outlet to which a pipe (G) is attached. Thep. 127water inlet pipe (H) passes through the lower end of the case (A). The electrodes (I, J) are secured, vertically, within the case, separated from each other equidistant, each alternate electrode being connected up with one wire (K), and the alternate electrodes with a wire (L).
Fig. 90. Electric Water PurifierFig. 90.Electric Water Purifier
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When the water passes upwardly, the decomposed or gaseous oxygen percolates through the water and thus attacks the germs and destroys them.
The Use of Hydrogen in Purification.—On the other hand, the hydrogen also plays an important part in purifying the water. This depends upon the material of which the electrodes are made. Aluminum is by far the best material, as it is one of nature's most active purifiers. All clay contains aluminum, in what is known as the sulphate form, and water passing through the clay of the earth thereby becomes purified, because of this element.
Aluminum Electrodes.—When this material is used as the electrodes in water, hydrate of aluminum is formed, or a compound of hydrogen and oxygen with aluminum. The product of decomposition is a flocculent matter which moves upwardly through the water, giving it a milky appearance. This substance is like gelatine, so that it entangles or enmeshes the germ life and prevents it from passing through a filter.
If no filter is used, this flocculent matter, as soon as it has given off the gases, will settle to the bottom and carry with it all decomposed matter, such as germs and other organic matter attackedp. 129by the oxygen, which has become entangled in the aluminum hydrate.
Electric Hand Purifier.—An interesting and serviceable little purifier may be made by any boy with the simplest tools, by cutting out three pieces of sheet aluminum. Hard rolled is best for the purpose. It is better to have one of the sheets (A), the middle one, thicker than the two outer plates (B).
Fig. 91. Portable Electric PurifierFig. 91.Portable Electric Purifier
Let each sheet be 1½ inches wide and 5½ inches thick. One-half inch from the upper ends of thep. 130two outside plates (B, B) bore bolt holes (C), each of these holes being a quarter of an inch from the edge of the plate. The inside plate (A) has two large holes (D) corresponding with the small holes (C) in the outside plates. At the upper end of this plate form a wing (E), ½ inch wide and ½ inch long, provided with a small hole for a bolt. Next cut out two hard-rubber blocks (F), each 1½ inches long, 1 inch wide and ⅜ inch thick, and then bore a hole (G) through each, corresponding with the small holes (C) in the plates (B). The machine is now ready to be assembled. If the inner plate is ⅛ inch thick and the outer plates each 1/16 inch thick, use two small eighth-inchp. 131bolts 1¼ inches long, and clamp together the three plates with these bolts. One of the bolts may be used to attach thereto one of the electric wires (H), and the other wire (I) is attached by a bolt to the wing (E).
Figs. 92-95. Details of Portable PurifierFigs. 92-95.Details of Portable Purifier
Such a device will answer for a 110-volt circuit, in ordinary water. Now fill a glass nearly full of water, and stand the purifier in the glass. Within a few minutes the action of electrolysis will be apparent by the formation of numerous bubbles on the plates, followed by the decomposition of the organic matter in the water. At first the flocculent decomposed matter will rise to the surface of the water, but before many minutes it will settle to the bottom of the glass and leave clear water above.
Purification and Separation of Metals.—This electrolytic action is utilized in metallurgy for the purpose of producing pure metals, but it is more largely used to separate copper from its base. In order to utilize a current for this purpose, a high ampere flow and low voltage are required. The sheets of copper, containing all of its impurities, are placed within a tank, parallel with a thin copper sheet. The impure sheet is connected with the positive pole of an electroplating dynamo, and the thin sheet of copper is connected with the negative pole. The electrolyte in the tank is ap. 132solution of sulphate of copper. The action of the current will cause the pure copper in the impure sheet to disintegrate and it is then carried over and deposited upon the thin sheet, this action continuing until the impure sheet is entirely eaten away. All the impurities which were in the sheet fall to the bottom of the tank.
Other metals are treated in the same way, and this treatment has a very wide range of usefulness.
Electroplating.—The next feature to be considered in electrolysis is a most interesting and useful one, because a cheap or inferior metal may be coated by a more expensive metal. Silver and nickel plating are brought about by this action of a current passing through metals, which are immersed in an electrolyte.
Plating Iron with Copper.—We have room in this chapter for only one concrete example of this work, which, with suitable modifications, is an example of the art as practiced commercially. Iron, to a considerable extent, is now being coated with copper to preserve it from rust. To carry out this work, however, an electroplating dynamo, of large amperage, is required, the amperage, of course, depending upon the surface to be treated at one time. The pressure should not exceed 5 volts
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The iron surface to be treated should first be thoroughly cleansed, and then immediately put into a tank containing a cyanide of copper solution. Two forms of copper solution are used, namely, the cyanide, which is a salt solution of copper, and the sulphate, which is an acid solution of copper. Cyanide is first used because it does not attack the iron, as would be the case if the sulphate solution should first come into contact with the iron.
A sheet of copper, termed the anode, is then placed within the tank, parallel with the surface to be plated, known as the cathode, and so mounted that it may be adjusted to or from the iron surface, or cathode. A direct current of electricity is then caused to flow through the copper plate and into the iron plate or surface, and the plating proceeded with until the iron surface has a thin film of copper deposited thereon. This is a slow process with the cyanide solution, so it is discontinued as soon as possible, after the iron surface has been completely covered with copper. This copper surface is thoroughly cleaned off to remove therefrom the saline or alkaline solution, and it is then immersed within a bath, containing a solution of sulphate of copper. The current is then thrown on and allowed sop. 134to remain until it has deposited the proper thickness of copper.
Direction of Current.—If a copper and an iron plate are put into a copper solution and connected up in circuit with each other, a primary battery is thereby formed, which will generate electricity. In this case, the iron will be positive and the copper negative, so that the current within such a cell would flow from the iron (in this instance, the anode) to the negative, or cathode.
The action of electroplating reverses this process and causes the current to flow from the copper to the iron (in this instance, the cathode).
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Generating Heat in a Wire.—When a current of electricity passes through a conductor, like a wire, more or less heat is developed in the conductor. This heat may be so small that it cannot be measured, but it is, nevertheless, present in a greater or less degree. Conductors offer a resistance to the passage of a current, just the same as water finds a resistance in pipes through which it passes. This resistance is measured in ohms, as explained in a preceding chapter, and it is this resistance which is utilized for electric heating.
Resistance of Substances.—Silver offers less resistance to the passage of a current than any other metal, the next in order is copper, while iron is, comparatively, a poor conductor.
The following is a partial list of metals, showing their relative conductivity:
Silver1.Copper1.04 to 1.09Gold1.38 to 1.41Aluminum1.64p. 136Zinc3.79Nickel4.69Iron6.56Tin8.9Lead13.2German Silver12.2 to 15
From this table it will be seen that, for instance, iron offers six and a half times the resistance of silver, and that German silver has fifteen times the resistance of silver.
This table is made up of strands of the different metals of the same diameters and lengths, so as to obtain their relative values.
Sizes of Conductors.—Another thing, however, must be understood. If two conductors of the same metal, having different diameters, receive the same current of electricity, the small conductor will offer a greater resistance than the large conductor, hence will generate more heat. This can be offset by increasing the diameter of the conductor. The metal used is, therefore, of importance, on account of the cost involved.
Comparison of Metals.—A conductor of aluminum, say, 10 feet long and of the same weight as copper, has a diameter two and a quarter times greater than copper; but as the resistance of aluminum is 50 per cent. more than that of silver, it will be seen that, weight for weight, copper isp. 137the cheaper, particularly as aluminum costs fully three times as much as copper.
Fig. 96. Simple Electric HeaterFig. 96.Simple Electric Heater
The table shows that German silver has the highest resistance. Of course, there are other metals, like antimony, platinum and the like, which have still higher resistance. German silver, however, is most commonly used, although there are various alloys of metal made which have high resistance and are cheaper.
The principle of all electric heaters is the same,p. 138namely, the resistance of a conductor to the passage of a current, and an illustration of a water heater will show the elementary principles in all of these devices.
A Simple Electric Heater.—In Fig.96the illustration shows a cup or holder (A) for the wire, made of hard rubber. This may be of such diameter as to fit upon and form the cover for a glass (B). The rubber should be ½ inch thick. Two holes are bored through the rubber cup, and through them are screwed two round-headed screws (C, D), each screw being 1½ inches long, so they will project an inch below the cap. Each screw should have a small hole in its lower end to receive a pin (E) which will prevent the resistance wire from slipping off.
The resistance wire (F) is coiled for a suitable length, dependent upon the current used, one end being fastened by wrapping it around the screw (C). The other end of the wire is then brought upwardly through the interior of the coil and secured in like manner to the other screw (D).
Caution must be used to prevent the different coils or turns from touching each other. When completed, the coil may be immersed in water, the current turned on, and left so until the water is sufficiently heated.
Fig. 97. Side view of resistance deviceFig. 97.Resistance Device
Fig. 98. Top view of resistance deviceFig. 98.Resistance Device
How to Arrange for Quantity of Currentp. 139Used.—It is difficult to determine just the proper length the coil should be, or the sizes of the wire, unless you know what kind of current you have. You may, however, rig up your own apparatus for the purpose of making it fit your heater, by preparing a base of wood (A) 8 inches long, 3 inches wide and 1 inch thick. On this mount four electric lamp sockets (B). Then connect the inlet wire (C) by means of short pieces of wire (D) with all the sockets on one side. The outlet wire (E) should then be connected up with the other sides of the sockets by the short wires (F). If, now, we have one 16-candlepower lamp in one of the sockets, there is a half ampere going through the wires (C, F). If there are two lampsp. 140on the board you will have 1 ampere, and so on. By this means you may readily determine how much current you are using and it will also afford you a means of finding out whether you have too much or too little wire in your coil to do the work.
Fig. 99. Plan View of Electric IronFig. 99.Plan View of Electric Iron
An Electric Iron.—An electric iron is made in the same way. The upper side of a flatiron has a circular or oval depression (A) cast therein, and a spool of slate (B) is made so it will fit into the depression and the high resistance wire (C) is wound around this spool, and insulating material, such as asbestos, must be used to pack around it. Centrally, the slate spool has an upwardly projecting circular extension (D) which passes through the cap or cover (E) of the iron. The wires of the resistance coil are then broughtp. 141through this circular extension and are connected up with the source of electrical supply. Wires are now sold for this purpose, which are adapted to withstand an intense heat.
Fig. 100. Section of Electric IronFig. 100.Section of Electric Iron
The foregoing example of the use of the current, through resistance wires, has a very wide application, and any boy, with these examples before him, can readily make these devices.
Thermo Electricity.—It has long been the dream of scientists to convert heat directly into electricity. The present practice is to use a boiler to generate steam, an engine to provide the motion, and a dynamo to convert that motion into electricity. The result is that there is loss in the process of converting the fuel heat into steam; loss to change the steam into motion, and loss top. 142make electricity out of the motion of the engine. By using water-power there is less actual loss; but water-power is not available everywhere.
Converting Heat Directly Into Electricity.—Heat may be converted directly into electricity without using a boiler, an engine or a dynamo, but it has not been successful from a commercial standpoint. It is interesting, however, to know and understand the subject, and for that reason it is explained herein.
Metals; Electric Positive-Negative.—To understand the principle, it may be stated that all metals are electrically positive-negative to each other. You will remember that it has hereinbefore been stated that if, for instance, iron and copper are put into an acid solution, a current will be created or generated thereby. So with zinc and copper, the usual primary battery elements. In all such cases an electrolyte is used.
Thermo-electricity dispenses with the electrolyte, and nothing is used but the metallic elements and heat. The word thermo means heat. If, now, we can select two strips of different metals, and place them as far apart as possible—that is, in their positive-negative relations with each other, and unite the end of one with one end of other by means of a rivet, and then heat the riveted ends, a current will be generated inp. 143the strips. If, for instance, we use an iron in conjunction with a copper strip, the current will flow from the copper to the iron, because copper is positive to iron, and iron negative to copper. It is from this that the term positive-negative is taken.
The two metals most available, which are thus farthest apart in the scale of positive-negative relation, are bismuth and antimony.
Fig. 101. Thermo-Electric CoupleFig. 101.Thermo-Electric Couple
In Fig.101is shown a thermo-electric couple (A, B) riveted together, with thin outer ends connected by means of a wire (C) to form a circuit. A galvanometer (D) or other current-testing means is placed in this circuit. A lamp is placed below the joined ends.
Thermo-Electric Couples.—Any number of these couples may be put together and joined at each end to a common wire and a fairly large flow of current obtained thereby.
One thing must be observed: A current willp. 144be generated only so long as there exists a difference in temperature between the inner and the outer ends of the bars (A, B). This may be accomplished by water, or any other cooling means which may suggest itself.
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Direct Current.—When a current of electricity is generated by a cell, it is assumed to move along the wire in one direction, in a steady, continuous flow, and is called adirectcurrent. This direct current is a natural one if generated by a cell.
Alternating Current.—On the other hand, the natural current generated by a dynamo is alternating in its character—that is, it is not a direct, steady flow in one direction, but, instead, it flows for an instant in one direction, then in the other direction, and so on.
A direct-current dynamo such as we have shown in Chapter IV, is much easier to explain, hence it is illustrated to show the third method used in generating an electric current.
It is a difficult matter to explain the principle and operation of alternating current machines, without becoming, in a measure, too technical for the purposes of this book, but it is important to know the fundamentals involved, so that the operation and uses of certain apparatus, like the chokingp. 146coil, transformers, rectifiers and converters, may be explained.
The Magnetic Field.—It has been stated that when a wire passes through the magnetic field of a magnet, so as to cut the lines of force flowing out from the end of a magnet, the wire will receive a charge of electricity.
Fig. 102. Cutting a Magnetic FieldFig. 102.Cutting a Magnetic Field
To explain this, study Fig.102, in which is a bar magnet (A). If we take a metal wire (B) and bend it in the form of a loop, as shown, and mount the ends on journal-bearing blocks, the wire may be rotated so that the loop will pass through the magnetic field. When this takes place, the wire receives a charge of electricity, which moves, say, in the direction of the darts, and will make a complete circuit if the ends of the looped wire are joined, as shown by the conductor (D).
Action of the Magnetized Wire.—You will remember, also that we have pointed out how, when a current passes over a wire, it has a magnetic field extending out around it at all points, so that while it is passing through the magnetic field ofp. 147the magnet (A), it becomes, in a measure, a magnet of its own and tries to set up in business for itself as a generator of electricity. But when the loop leaves the magnetic field, the magnetic or electrical impulse in the wire also leaves it.
The Movement of a Current in a Charged Wire.—Your attention is directed, also, to another statement, heretofore made, namely, that when a current from a charged wire passes by induction to a wire across space, so as to charge it with an electric current, it moves along the charged wire in a direction opposite to that of the current in the charging wire.
Now, the darts show the direction in which the current moves while it is approaching and passing through the magnetic field. But the moment the loop is about to pass out of the magnetic field, the current in the loop surges back in the opposite direction, and when the loop has made a revolution and is again entering the magnetic field, it must again change the direction of flow in the current, and thus produce alternations in the flow thereof.
Let us illustrate this by showing the four positions of the revolving loop. In Fig.103the loop (B) is in the middle of the magnetic field, moving upwardly in the direction of the curved dart (A), and while in that position the voltage, or thep. 148electrical impulse, is the most intense. The current used flows in the direction of the darts (C) or to the left.
In Fig.104, the loop (A) has gone beyond the influence of the magnetic field, and now the current in the loop tries to return, or reverse itself, as shown by the dart (D). It is a reaction that causes the current to die out, so that when the loop has reached the point farthest from the magnet, as shown in Fig.105, there is no current in the loop, or, if there is any, it moves faintly in the direction of the dart (E).
Fig. 13-106. Illustrating AlternationsFig. 103-106.Illustrating Alternations
Current Reversing Itself.—When the loop reaches its lowest point (Fig.106) it again comes within the magnetic field and the current commences to flow back to its original direction, as shown by darts (C)
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Self-Induction.—This tendency of a current to reverse itself, under the conditions cited, is called self-induction, or inductance, and it would be well to keep this in mind in pursuing the study of alternating currents.
You will see from the foregoing, that the alternations, or the change of direction of the current, depends upon the speed of rotation of the loop past the end of the magnet.
Fig. 107. Form for Increasing AlternationsFig. 107.Form for Increasing Alternations
Fig. 108. Form for Increasing AlternationsFig. 108.Form for Increasing Alternations
Instead, therefore, of using a single loop, we may make four loops (Fig.107), which at the same speed as we had in the case of the single loop, will give four alternations, instead of one, and still further, to increase the periods of alternation, we may use the four loops and two magnets,p. 150as in Fig.108. By having a sufficient number of loops and of magnets, there may be 40, 50, 60, 80, 100 or 120 such alternating periods in each second. Time, therefore, is an element in the operation of alternating currents.
Let us now illustrate the manner of connecting up and building the dynamo, so as to derive the current from it. In Fig.109, the loop (A) shows, for convenience, a pair of bearings (B). A contact finger (C) rests on each, and to these the circuit wire (D) is attached. Do not confuse these contact fingers with the commutator brushes, shown in the direct-current motor, as they are there merely for the purpose of making contact between the revolving loop (A) and stationary wire (D).
Fig. 109. Connection of Alternating Dynamo ArmatureFig. 109.Connection of Alternating Dynamo Armature
Brushes in a Direct-Current Dynamo.—The object of the brushes in the direct-current dynamo, in connection with a commutator, is to convert thisinductanceof the wire, or this effort to reverse itself into a current which will go in onep. 151direction all the time, and not in both directions alternately.
To explain this more fully attention is directed to Figs.110and111. Let A represent the armature, with a pair of grooves (B) for the wires. The commutator is made of a split tube, the parts so divided being insulated from each other, and in Fig.110, the upper one, we shall call and designate the positive (+) and the lower one the negative (-). The armature wire (C) has one end attached to the positive commutator terminal and the other end of this wire is attached to the negative terminal.