Things a Boy Should Know About Electricity / Second Edition

124. Induction Coils in Telephone Work. As the resistance of long telephone lines is great, a high electrical pressure, or E.M.F. is desired. While the current from one or two cells is sufficient to work the transmitter properly, and cause undulating currents in the short line, it does not have power enough to force its way over a long line.

To get around this difficulty, an induction coil, Fig. 133, is used to transform the battery current, that flows through the carbon transmitter and primary coil, into a current with a high E. M. F. The battery current in the primary coil is undulating, but always passes in the same direction, making the magnetic field around the core weaker and stronger. This causes an alternating current in the secondary coil and main line. In Fig. 133 P and S represent the primary and secondary coils. P is joined in series with a cell and carbon transmitter; S is joined to the distant receiver. One end of S can be grounded, the current completing the circuit through the earth and into the receiver through another wire entering the earth.

125. Various forms of telephones are shown in Figs. 134, 135, 136. Fig. 134 shows a form of desk telephone; Fig. 135 shows a common form of wall telephone; Fig. 136 shows head-telephones for switchboard operators.

CHAPTER XVII.
HOW ELECTRICITY IS GENERATED BY DYNAMOS.

126. The Dynamo, Dynamo-Electric Machine or Generator, is a machine for converting mechanical energy into an electric current, through electromagnetic induction. The dynamo is a machine that will convert steam power, for example, into an electric current. Strictly speaking, a dynamo creates electrical pressure, or electromotive force, and not electricity, just as a force-pump creates water-pressure, and not water. They are generally run by steam or water power.

127. Induced Currents. We have already spoken about currents being induced by moving a coil of wire in a magnetic field. We shall now see how this principle is used in the dynamo which is a generator of induced currents.

Fig. 137 shows how a current can be generated by a bar magnet and a coil of wire. Fig. 138 shows how a current can be generated by a horseshoe magnet and a coil of wire having an iron core. The ends of the coil are to be connected to an astatic galvanoscope; this forms a closed circuit. The coil may be moved past the magnet, or the magnet past the coil.

Fig. 139 shows how a current can be generated by two coils, H being connected to an astatic galvanoscope and E to a battery. By suddenly bringing E toward H or the core of E past that of H, a current is produced. We have in this arrangement the main features of a dynamo. We can reverse the operation, holding E in one position and moving H rapidly toward it. In this case H would represent the armature and E the field-magnet. When H is moved toward E, the induced current in H flows in one direction, and when H is suddenly withdrawn from E the current is reversed in H. (See "Study," Chapter XXV., for experiments.)

128. Induced Currents by Rotary Motion. The motions of the coils in straight lines are not suitable for producing currents strong enough for commercial purposes. In order to generate currents of considerable strength and pressure, the coils of wire have to be pushed past magnets, or electromagnets, with great speed. In the dynamo the coils are so wound that they can be given a rapid rotary motion as they fly past strong electromagnets. In this way the coil can keep on passing the same magnets, in the same direction, as long as force is applied to the shaft that carries them.

129. Field-Magnets; Armature; Commutator. What we need then, to produce an induced current by a rotary motion, is a strong magnetic field, a rotating coil of wire properly placed in the field, and some means of leading the current from the machine.

If a loop of wire, Fig. 140, be so arranged on bearings at its ends that it can be made to revolve, a current will flow through it in one direction during one-half of the revolution, and in the opposite direction during the other half, it being insulated from all external conductors. This agrees with the experiments suggested in § 127, when the current generated in a coil passed in one direction during its motion toward the strongest part of the field, and in the opposite direction when the coil passed out of it. A coil must be cut by lines of force to generate a current. A current inside of the machine, as in Fig. 140, would be of no value; it must be led out to external conductors where it can do work. Some sort of sliding contact is necessary to connect a revolving conductor with outside stationary ones. The magnet, called the field-magnet, is merely to furnish lines of magnetic force. The one turn of wire represents the simplest form of armature.

Fig. 141 shows the ends of a coil joined to two rings, X, Y, insulated from each other, and rotating with the coil. The two stationary pieces of carbon, A, B, called brushes, press against the rings, and to these are joined wires, which complete the circuit, and which lead out where the current can do work. The arrows show the direction of the current during one-half of a revolution. The rings form a collector, and this arrangement gives an alternating current.

In Fig. 142 the ends of the coil are joined to the two halves of a cylinder. These halves, X and Y, are insulated from each other, and from the axis. The current flows from X onto the brush A, through some external circuit, to do the work, and thence back through brush B onto Y. By the time that Y gets around to A, the direction of the current in the loop has reversed, so that it passes toward Y, but it still enters the outside circuit through A, because Y is then in contact with A. This device is called a commutator, and it allows a constant or direct current to leave the machine.

In regular machines, the field-magnets are electromagnets, the whole or a part of the current from the dynamo passing around them on its way out, to excite them and make a powerful field between the poles. To lessen the resistance to the lines of force on their way from the N to the S pole of the field-magnets, the armature coils are wound on an iron core; this greatly increases the strength of the field, as the lines of force have to jump across but two small air-gaps. There are many loops of wire on regular armatures, and many segments to the commutator, carefully insulated from each other, each getting its current from the coil attached to it.

130. Types of Dynamos. While there is an almost endless number of different makes and shapes of dynamos, they may be divided into two great types; the continuous or direct current, and the alternating current dynamo. Direct current machines give out a current which constantly flows in one direction, and this is because a commutator is used. Alternating currents come from collectors or rings, as shown in Fig. 141; and as an alternating current cannot be used to excite the fields, an outside current from a small direct current machine must be used. These are called exciters.

In direct current machines enough residual magnetism is left in the field to induce a slight current in the armature when the machine is started. This immediately adds strength to the field-magnets, which, in turn, induce a stronger current in the armature.

131. Winding of Dynamos. There are several ways of winding dynamos, depending upon the special uses to be made of the current.

The series wound dynamo, Fig. 143, is so arranged that the entire current passes around the field-magnet cores on its way from the machine. In the shunt wound dynamo, Fig. 144, a part, only, of the current from the machine is carried around the field-magnet cores through many turns of fine wire. The compound wound dynamo is really a combination of the two methods just given. In separately-excited dynamos, the current from a separate machine is used to excite the field-magnets.

132. Various Machines. Fig. 145 shows a hand power dynamo which produces a current for experimental work. Fig. 146 shows a magneto-electrical generator which produces a current for medical use. Figs. 147, 148 show forms of dynamos, and Fig. 149 shows how arc lamps are connected in series to dynamos.

CHAPTER XVIII.
HOW THE ELECTRIC CURRENT IS TRANSFORMED.

133. Electric Current and Work. The amount of work a current can do depends upon two factors; the strength (amperes), and the pressure, or E. M. F. (volts). A current of 10 amperes with a pressure of 1,000 volts = 10 × 1,000 = 10,000 watts. This furnishes the same amount of energy as a current of 50 amperes at 200 volts; 50 × 200 = 10,000 watts.

134. Transmission of Currents. It is often necessary to carry a current a long distance before it is used. A current of 50 amperes would need a copper conductor 25 times as large (sectional area) as one to carry the 10 ampere current mentioned in § 133. As copper conductors are very expensive, electric light companies, etc., generally try to carry the current on as small a wire as possible. To do this, the voltage is kept high, and the amperage low. Thus, as seen in § 133, the current of 1,000 volts and 10 amperes could be carried on a much smaller wire than the other current of equal energy. A current of 1,000 volts, however, is not adapted for lights, etc., so it has to be changed to lower voltage by some form of transformer before it can be used.

135. Transformers, like induction coils, are instruments for changing the E. M. F. and strength of currents. There is very little loss of energy in well-made transformers. They consist of two coils of wire on one core; in fact, an induction coil may be considered a transformer, but in this a direct current has to be interrupted. If the secondary coil has 100 times as many turns of wire as the primary, a current of 100 volts can be taken from the secondary coil when the primary current is but 1 volt; but the strength (amperes) of this new current will be but one-hundredth that of the primary current.

By using the coil of fine wire as the primary, we can lower the voltage and increase the strength in the same proportion.

Fig. 150 shows about the simplest form of transformer with a solid iron core, on which are wound two coils, the one, P, being the primary, and the other, S, the secondary. Fig. 151 shows the general appearance of one make of transformer. The operation of this apparatus, as already mentioned, is to reduce the high pressure alternating current sent out over the conductors from the dynamo, to a potential at which it can be employed with convenience and safety, for illumination and other purposes. They consist of two or more coils of wire most carefully insulated from one another. A core or magnetic circuit of soft iron, composed of very thin punchings, is then formed around these coils, the purpose of the iron core being to reduce the magnetic resistance and increase the inductive effect. One set of these coils is connected with the primary or high-pressure wires, while the other set, which are called the secondary coils, is connected to the house or low-pressure wires, or wherever the current is required for use. The rapidly alternating current impulses in the primary or high-pressure wires induce secondary currents similar in form but opposite in direction in the secondary coils. These current impulses are of a much lower pressure, depending upon the ratio of the number of turns of wire in the respective coils, it being customary to wind transformers in such a manner as to reduce from 1,000 or 2,000-volt primaries to 50 or 100-volt secondaries, at which voltage the secondary current is perfectly harmless.

136. Motor-Dynamos. Fig. 152. These consist essentially of two belt-type machines on a common base, direct coupled together, one machine acting as a motor to receive current at a certain voltage, and the other acting as a dynamo to give out the current usually at a different voltage. As they transform current from one voltage to another, motor-dynamos are sometimes called Double Field Direct Current Transformers. The larger sizes have three bearings, one bearing being between the two machines, while the smaller sizes have but two bearings, the two armatures being fastened to a common spider.

Applications. The uses to which motor-dynamos are put are very various. They are extensively used in the larger sizes as "Boosters," for giving the necessary extra force on long electric supply circuits to carry the current to the end with the same pressure as that which reaches the ends of the shorter circuits from the station.

Motor-dynamos have the advantage over dynamotors, described later, of having the secondary voltage easily and economically varied over wide ranges by means of a regulator in the dynamo field.

137. Dynamotors. Fig. 153. In Dynamotors the motor and dynamo armatures are combined in one, thus requiring a single field only. The primary armature winding, which operates as a motor to drive the machine, and the secondary or dynamo winding, which operates as a generator to produce a new current, are upon the same armature core, so that the armature reaction of one winding neutralizes that of the other. They therefore have no tendency to spark, and do not require shifting of the brushes with varying load. Having but one field and two bearings, they are also more efficient than motor-dynamos.

CHAPTER XIX.
HOW ELECTRIC CURRENTS ARE DISTRIBUTED FOR USE.

138. Conductors and Insulators. To carry the powerful current from the generating station to distant places where it is to give heat, power, or light, or even to carry the small current of a single cell from one room to another, conductors must be used. To keep the current from passing into the earth before it reaches its destination insulators must be used. The form of conductors and insulators used will depend upon the current and many other conditions. It should be remembered that the current has to be carried to the lamp or motor, through which it passes, and then back again to the dynamo, to form a complete circuit. A break anywhere in the circuit stops the current. Insulators are as important as conductors.

139. Mains, Service Wires, etc. From the switchboard the current flows out through the streets in large conductors, or mains, the supply being kept up by the dynamos, just as water-pressure is kept up by the constant working of pumps. Branches, called service wires, are led off from the mains to supply houses or factories, one wire leading the current into the house from one main, and a similar one leading it out of the house again to the other main.

In large buildings, pairs of wires, called risers, branch out from the service wires and carry the current up through the building. These have still other branches—floor mains, etc., that pass through halls, etc., smaller branches finally reaching the lamps. The sizes of all of these wires depend upon how much current has to pass through them. The mains in large cities are usually placed underground. In some places they are carried on poles.

140. Electric Conduits are underground passages for electric wires, cables, etc. There are several ways of insulating the conductors. Sometimes they are placed in earthenware or iron tubes, or in wood that has been treated to make it water-proof. At short distances are placed man-holes, where the different lengths are joined, and where branches are attached.

Fig. 154 shows creosoted wooden pipes; Fig. 155 shows another form of wooden pipe. Fig. 156 shows a coupling-box used to join Edison tubes. The three wires, used in the three-wire system, are insulated from each other, the whole being surrounded by an iron pipe of convenient length for handling. Fig. 157 shows sections of man-holes and various devices used in conduit work.

141. Miscellaneous Appliances. When the current enters a house for incandescent lighting purposes, for example, quite a number of things are necessary. To measure the current a meter is usually placed in the cellar. In new houses the insulated conductors are usually run through some sort of tube which acts as a double protection, all being hidden from view. Fig. 158 shows a short length of iron tube with a lining of insulating material. Wires are often run through tubes made of rubber and various other insulating materials.

Where the current is to be put into houses after the plastering has been done, the wires are usually run through mouldings or supported by cleats. Fig. 159 shows a cross-section of moulding. The insulated wires are placed in the slots, which are then covered.

Fig. 160 shows a form of porcelain cleat. These are fastened to ceilings or walls, and firmly hold the insulated wires in place. Fig. 161 shows a wood cleat. Fig. 162 shows small porcelain insulators. These may be screwed to walls, etc., the wire being then fastened to them. Fig. 163 shows how telegraph wires are supported and insulated. Fig. 164 shows how wires may be carried by tree and insulated from them.

142. Safety Devices. We have seen that when too large a current passes through a wire, the wire becomes heated and may even be melted. Buildings are wired to use certain currents, and if from any cause much more current than the regular amount should suddenly pass through the service wires into the house, the various smaller wires would become overheated, and perhaps melt or start a fire. An accidental short circuit, for example, would so reduce resistance that too much current would suddenly rush through the wires. There are several devices by which the over-heating of wires is obviated.

Fig. 165 shows a safety fuse, or safety cut-out, which consists of a short length of easily fusible wire, called fuse wire, placed in the circuit and supported by a porcelain block. These wires are tested, different sizes being used for different currents. As soon as there is any tendency toward over-heating, the fuse blows; that is, it promptly melts and opens the circuit before any damage can be done to the regular conductors. Fig. 166 shows a cross-section of a fuse plug that can be screwed into an ordinary socket. The fuse wire is shown black.

Fig. 167 shows a fuse link. These are also of fusible material, and so made that they can be firmly held under screw-heads. For heavy currents fuse ribbons are used, or several wires or links may be used side by side. Fig. 168 shows a fusible rosette. Fig. 169 shows two fuse wires fixed between screw-heads, the current passing through them in opposite directions, both sides of the circuit being included. Fig. 170 shows various forms of cut-outs.

143. Wires and Cables are made in many sizes. Figs. 171 to 175 show various ways of making small conductors. They are made very flexible, for some purposes, by twisting many small copper wires together, the whole being then covered with insulating material.

144. Lamp Circuits. As has been noted before, in order to have the electric current do its work, we must have a complete circuit. The current must be brought back to the dynamo, much of it, of course, having been used to produce light, heat, power, etc. For lighting purposes this is accomplished in two principal ways.

Fig. 178 shows a number of lamps so arranged, "in series," that the same current passes through them all, one after the other. The total resistance of the circuit is large, as all of the lamp resistances are added together.

Fig. 179 shows lamps arranged side by side, or "in parallel," between the two main wires. The current divides, a part going through each lamp that operates. The total resistance of the circuit is not as large as in the series arrangement, as the current has many small paths in going from one main wire to the other. Fig. 179 also shows the ordinary two-wire system for incandescent lighting, the two main wires having usually a difference of potential equal to 50 or 110 volts. These comparatively small pressures require fairly large conductors.

The Three-Wire System, Fig. 180, uses the current from two dynamos, arranged with three main wires. While the total voltage is 220, one of the wires being neutral, 110 volts can be had for ordinary lamps. This voltage saves in the cost of conductors.

The Alternating System, Fig. 181, uses transformers. The high potential of the current allows small main wires, from which branches can be run to the primary coil of the transformer. The secondary coil sends out an induced current of 50 or 110 volts, while that in the primary may be 1,000 to 10,000 volts.

CHAPTER XX.
HOW HEAT IS PRODUCED BY THE ELECTRIC CURRENT.

145. Resistance and Heat. We have seen that all wires and conductors offer resistance to the electric current. The smaller the wire the greater its resistance. Whenever resistance is offered to the current, heat is produced. By proper appliances, the heat of resistance can be used to advantage for many commercial enterprises. Dynamos are used to generate the current for heating and lighting purposes.

146. Electric Welding. Fig. 183 shows one form of electric welding machine. The principle involved in the art of electric welding is that of causing currents of electricity to pass through the abutting ends of the pieces of metal which are to be welded, thereby generating heat at the point of contact, which also becomes the point of greatest resistance, while at the same time mechanical pressure is applied to force the parts together. As the current heats the metal at the junction to the welding temperature, the pressure follows up the softening surface until a complete union or weld is effected; and, as the heat is first developed in the interior of the parts to be welded, the interior of the joint is as efficiently united as the visible exterior. With such a method and apparatus, it is found possible to accomplish not only the common kinds of welding of iron and steel, but also of metals which have heretofore resisted attempts at welding, and have had to be brazed or soldered.

The introduction of the electric transformer enables enormous currents to be so applied to the weld as to spend their energy just at the point where heating is required. They need, therefore, only to be applied for a few seconds, and the operation is completed before the heat generated at the weld has had time to escape by conduction to any other part.

Although the quantity of the current so employed in the pieces to be welded is enormous, the potential at which it is applied is extremely low, not much exceeding that of the batteries of cells used for ringing electric bells in houses.

147. Miscellaneous Applications. Magneto Blasting Machines are now in very common use for blasting rocks, etc. Fig. 184 shows one, it being really a small hand dynamo, occupying less than one-half a cubic foot of space. The armature is made to revolve rapidly between the poles of the field-magnet by means of a handle that works up and down. The current is carried by wires from the binding-posts to fuses. The heat generated by resistance in the fuse ignites the powder or other explosive.

Electric soldering irons, flat-irons, teakettles, griddles, broilers, glue pots, chafing-dishes, stoves, etc., etc., are now made. Figs. 185 to 189 show some of these applications. The coils for producing the resistance are inclosed in the apparatus.

Fig. 190 shows a complete electric kitchen. Any kettle or part of the outfit can be made hot by simply turning a switch. Fig. 191 shows an electric heater placed under a car seat. Many large industries that make use of the heating effects of the current are now being carried on.

CHAPTER XXI.
HOW LIGHT IS PRODUCED BY THE INCANDESCENT LAMP.

148. Incandescence. We have just seen that the electric current produces heat when it flows through a conductor that offers considerable resistance to it. As soon as this was discovered men began to experiment to find whether a practical light could also be produced. It was found that a wire could be kept hot by constantly passing a current through it, and that the light given out from it became whiter and whiter as the wire became hotter. The wire was said to be incandescent, or glowing with heat. As metal wires are good conductors of electricity, they had to be made extremely fine to offer enough resistance; too fine, in fact, to be properly handled.

149. The Incandescent Lamp. Many substances were experimented upon to find a proper material out of which could be made a filament that would give the proper resistance and at the same time be strong and lasting. It was found that hair-like pieces of carbon offered the proper resistance to the current. When heated in the air, however, carbon burns; so it became necessary to place the carbon filaments in a globe from which all the air had been pumped before passing the current through them. This proved to be a success.

Fig. 192 shows the ordinary form of lamp. The carbon filament is attached, by carbon paste, to short platinum wires that are sealed in the glass, their lower ends being connected to short copper wires that are joined to the terminals of the lamp. When the lamp is screwed into its socket, the current can pass up one side of the filament and down the other. The filaments used have been made of every form of carbonized vegetable matter. Bamboo has been largely used, fine strips being cut by dies and then heated in air-tight boxes containing fine carbon until they were thoroughly carbonized. This baking of the bamboo produces a tough fiber of carbon. Various forms of thread have been carbonized and used. Filaments are now made by pressing finely pulverized carbon, with a binding material, through small dies. The filaments are made of such sizes and lengths that will adapt them to the particular current with which they are to be used. The longer the filament, the greater its resistance, and the greater the voltage necessary to push the current through it.

After the filaments are properly attached, the air is pumped from the bulb or globe. This is done with some form of mercury pump, and the air is so thoroughly removed from the bulb that about one-millionth only of the original air remains. Before sealing off the lamp, a current is passed through the filament to drive out absorbed air and gases, and these are carried away by the pump. By proper treatment the filaments have a uniform resistance throughout, and glow uniformly when the current passes.

150. Candle-Power. A lamp is said to have 4, 8, 16 or more candle-power. A 16-candle-power lamp, for example, means one that will give as much light as sixteen standard candles. A standard sperm candle burns two grains a minute. The candle-power of a lamp can be increased by forcing a strong current through it, but this shortens its life.

The Current used for incandescent lamps has to be strong enough to force its way through the filament and produce a heat sufficient to give a good light. The usual current has 50 or 110 volts, although small lamps are made that can be run by two or three cells. If the voltage of the current is less than that for which the lamp was made, the light will be dim. The filament can be instantly burned out by passing a current of too high pressure through it.

Even with the proper current, lamps soon begin to deteriorate, as small particles of carbon leave the filament and cling to the glass. This is due to the evaporation, and it makes the filament smaller, and a higher pressure is then needed to force the current through the increased resistance; besides this, the darkened bulb does not properly let the light out. The current may be direct or alternating.

151. The Uses to which incandescent lamps are put are almost numberless. Fig. 193 shows a decorative lamp. Fancy lamps are made in all colors. Fig. 194 shows a conic candle lamp, to imitate a candle. What corresponds to the body of the candle (see figure B to C) is a delicately tinted opal glass tube surmounted (see figure A to B) by a finely proportioned conic lamp with frosted globe. C to D in the figure represents the regular base, and thus the relative proportions of the parts are shown. Fig. 195 shows another form of candelabra lamp. Fig. 196 shows small dental lamps. Fig. 197 shows a small lamp with mirror for use in the throat. Fig. 198 shows lamp with half shade attached, used for library tables. Fig. 199 shows an electric pendant for several lamps, with shade. Fig. 200 shows a lamp guard. Fig. 201 shows a lamp socket, into which the lamp is screwed. Fig. 202 shows incandescent bulbs joined in parallel to the + and - mains. Fig. 203 shows how the lamp cord can be adjusted to desired length. Fig. 204 shows a lamp with reflector placed on a desk. Fig. 205 shows a form of shade and reflector.