ELECTRICITY In large coils the secondary is wound in flat spirals, well insulated from each other, as the difference of potential between adjacent wires is then not as large as when the coil is wound in layers from end to end. The primary is insulated from the secondary by a glass or rubber tube, and the turns of the secondary well insulated from each other, by boiling in melted paraffine or wax. Spottiswood constructed a coil, the secondary of which consisted of 280 miles of wire wound in 340,000 turns, and gave a discharge through 421⁄2 inches of air. A current which is rapidly alternating in direction may be used in the primary, but in that case the circuit-breaker is not used. (See § 76.) $75. TESLA'S INDUCTION COIL. Since the discharge of a condenser is oscillatory (see §.92), the rapidly alternating current thus produced, when sent through the primary of a properly constructed induction-coil, induces E. M. F.'s of enormous values; the discharge under these conditions is extremely beautiful and interesting. (See $94.) While the above principle has long been known, we are indebted to Tesla for its successful application. The electrical connections for securing the above conditions are shown in Fig. 56. I is an ordinary Ruhmkorff induction-coil, the interrupter of which is not shown. The terminals 1155 side the secondary, and the terminals of both are brought to the top of the case through hard rubber tubes. The whole is then immersed in oil in a zinc-covered wooden box. The action of the coil depends largely upon the proper adjustment of the capacity, self-induction and resistance in the circuit. $76. TRANSFORMER. A transformer is an induction-coil for the purpose of changing an alternating current, usually from a high to a low potential, though the reverse is sometimes the case. of the secondary SS are connected with the condenser, C; the terminals of the condenser are connected to the primary, P, of the Tesla, or disruptive discharge coil as it is often called. G is a sparkgap consisting of two polished metal balls a small distance apart. S, is the secondary of the Tesla coil. When the current in the primary, P, is broken a high E. M. F. is induced in the secondary S; this charges the condenser until the potential is sufficient to overcome the resistance of the gap, G. The air then breaks down, and the oscillatory discharge of the condenser takes place through the primary, P,. The construction of the Tesla coil is shown in Fig. 57. PP is the primary, wound oppositely, in two parts. In this particular coil each half of the primary is composed of four layers of twentyfour turns each, or ninety-six turns. The secondary, SS, is also in two parts of twentyfour layers, ten turns each, or 260 turns for each half. The two halves are wound oppositely and connected in series; the secondary is wound on two spools of hard rubber, RR, held apart by the hard rubber rods, C. The primary is placed inThe primary is placed in Fig. 57. In the induction-coils previously described the object has been to transform a low to a high potential. Faraday's ring (Fig. 54) is the earliest and the type of all transformers, and the electromotive force generated in the secondary circuit is to that employed in the primary very nearly as the relative number of turns in the two coils; if the primary has fewer turns than the secondary, it is called a "step-up'' transformer, and a "stepdown" transformer when the primary has greater number of turns. Fig. 58 shows the coils and magnetic circuit of one of the many commercial types of transformers. It will be noticed that the magnetic circuit. is entirely of iron, the iron being built up of thin sheets separated by varnish or simply the oxide on the plates, in order that there may be no electric currents set up by induction in the iron core. The direction of the plate is arranged perpendicular to that in Fig. 58. would be, for the first case (5) X 2 = 50 watts; while in the second case it would be (10) x 2 = 200; therefore it would be more economical to use 5 amperes at 1,000 volts. There is a limit, however, to the extent this principle may be applied. It is not practical to use a voltage as high as 1,000 for incandescent lighting, on account of the danger to life, leakage, and difficulties attending the manufacture of the lamps; hence, when the electrical energy is to be distributed over distances of as much as one or two miles, the current is generated as an alternating current at, say, 1,000 volts, and changed, by means of a transformer, to a potential of 100 volts before it enters the house or place where it is to be used. The loss in transformation is slight if the transformer is properly constructed. When very great distances are to be overcome, the current is generated at as high a potential as is practical; it is then transformed to a still higher potential by means of a step-up transformer, transmitted at this potential, and changed back again by a step-down transformer at the other end of the line. 877. ROTARY TRANSFORMERS. A rotary transformer is not, strictly speaking, an inductioncoil, but a dynamo and motor combined. The armature has two sets of windings, one of which receives the current to be transformed, and drives the machine as a motor; the other set of wind The regulation of a transformer is an interesting case of self and mutual induction. The primary remains in connection with the mains, yet very little current flows through it unless the sec-ings on the armature generates a current at the ondary is closed; furthermore, the current supplied by the secondary is the correct amount needed for the number of lamps turned on. Consider one of the transformers represented in Fig. 59, and let the secondary circuit be open; the reverse E. M. F. in P due to its self-induction almost equals the E. M. F. supplied; hence but little current flows through P. If, now, the secondary be closed, with, say, one lamp in circuit, there will be mutual induction, and this acts in opposition to the self-induction in the primary; thus more of the E. M. F. impressed upon the primary from the mains is allowed to act; or in other words, the mutual induction due to a current in the secondary opposes the self-induction of the primary almost in proportion to the current flowing in the secondary. If more lamps are turned on, the current in the primary regulates itself to suit the load. The rate at which electrical energy is transformed into heat in a conductor is I2R (§ 43); therefore, if the current is doubled, the loss in the conductor is four times as great, or varies as to the square of the current. Since the energy of a current is also IV, or the strength of current times the fall of potential, the factors I and V may be varied, and the energy transmitted remain the same. For example, if I is five amperes, and V 1,000 volts, the rate at which energy is transmitted is 5,000 watts; also, if I is ten amperes, and V 500 volts, the energy rate is 5,000. Now, if this energy is to be transmitted to a distance, through a conductor having a resistance of 2 ohms, the loss in the conductor potential desired. Both armatures have a common axis, and rotate in the same magnetic field. $78. CHOKING-COIL. A choking-coil is one having large self-induction, but little resistance, and used as a resistance for alternating-currents. The primary of a transformer is a good example of such a coil, when the secondary circuit is open. They are usually constructed so that the selfinduction may be varied by varying the magnetic field, or the number of turns in the coil. Such a coil has the property of cutting down the current, but without the expenditure of energy which would take place were a simple resistance used. $79. DYNAMO ELECTRIC-MACHINES. Any treatment of the subject of electromagnetic induction would be incomplete without at least the elementary theory of the dynamo electric machine, which is by far the most important application of this principle. In the following articles no attempt is made to cover any but the most important and essential features of typical forms. A dynamo electric-machine, or simply dynamo, may be defined as a machine for converting me chanical energy into the energy of an electric current; it consists of a series of conducting cir cuits, so arranged that the magnetic flow through them is continually changing. Figure 60 is a diagram of a typical dynamo. A is the armature, made of soft iron, and carrying on its surface the conductors through which the magnetic flow is made to change, and in which the current is generated. S and N are the poles of an electromagnet, F, called the field-magnet; C is the col ELECTRICITY magnetic flow through the circuit varies as the cosine of the angle revolved through. The electromotive force generated in the circuit at any time does not depend on the flow, but the rate at which the flow is changing. At position 1, there is a large number of lines through the circuit, but if it be moved through a small angle, CO1, the number of lines is nearly as great as before, while at 3 the same small angle C'03 would produce a large change in the flow, hence the E. M. F. will be o at and 5, and a positive maximum at 3, and a negative maximum at 7. Let the circle (Fig. 63) be divided up into a great number of small angle parts do, and let dt be the time required for the d conductor to revolve through this angle do. The E. M. F. dF generated at any time is dt where dF is the change in the number of lines through the circuit in the small time, dt; ab de a Fig. 63. may be considered as a straight line and proportional to dt, and ac is proportional to dF; hence the E. M. F. is E = E. M. F. varies as the placement. аС aO = ab ad sin 0; hence the sine of the angle of dis These relations can best be represented by curves, as in Fig. 64. The horizontal distances At position 1, the circuit incloses the maximum number of lines; at position 2, the number of lines is proportional to the line Oa, the exact number being Fcos 0, F being the maximum flow, and the angle moved through. At the position 3, the circuit does not inclose any lines; at position 4, the lines thread through from the opposite side of the loop, which is equivalent to a decrease in the flow from a maximum at 1 to a maximum at 5 in the other direction. At 7, the flow is again O, and increases to the maximum at 1; hence the Fig. 64. represent equal increments of time begun at I (Fig. 62); the vertical distances of the curve I are the values of the cosine for the different positions of the conductor, and represent the changes in the magnetic flow. The corresponding values of the sines are shown in curve II, the ordinates of which represent the induced E.M.F. The maximum values differ by 90°, or a quarter of a period. The current in such a circuit changes direction to correspond with the change in direction of the E. M. F.; such a current is called an alternating current. In case it is desired to send the current always in the same direction, the coil terminates in two parts of a split tube, a and (Fig. 65) insulated from each other, and from the shaft; the brushes, and d, are placed so that they exchange contacts the same instant that the circuit reverses, thus sending the current in d the same direction, but Fig. 65. varied as before, all the curves lying above the axis, as in Fig. 66. с a C N 3 tinuous current-machines. Its construction is shown diagrammatically in Fig. 67. The core, C, is a laminated iron ring; the wires are wound on the ring in one continuous spiral. In the figure External the spiral is divided into eight coils, the end of one coil connected to the beginning of the next, as well as to one of the collector-bars, which are insulated. from the shaft and from each other. The current in coils 1, 2, 3 and 4 will be in the direction indicated by the arrows, and out through the brush, B, to the external circuits and back to the brush, B,. the current is found to be opposite coils 5, 6, 7 and 8 are examined, in direction to the coils on the right half, but both currents have the common outlet, B, and inlet, B,. The coils of each half are in series, and the two halves are in parallel, as shown in Fig. 68. Fig. 68. If the Fig. 70. a laminated cylinder. Fig. 70 shows the construction of a drum armature. 82. FIELD-MAGNETS. The field-magnets of a dynamo are usually made of good cast-iron. Cast-iron is not as permeable as wrought-iron, but the former is much more easily shaped, and since the armature requires a certain amount of space for the required number and size of conductors, the cross-section of the field is usually of such a size that the cast-iron cores may be enlarged enough to compensate for the poor per meability. net coils are made of wrought-iron, but united at In many machines the cores of the field-magthe base by the cast-iron frame of the machine, and capped by cast-iron pole-pieces in order to get the required cross-section of field. The field-magnets are excited by the current of the machine; the method of winding the coils depending upon the use to which the machine is to be put. In all cases there is enough residual magnetism to start the machine. current leaves the upper brush, B, and is all sent Fig. 60 represents a series-wound machine. The through the coils of the field-magnets; that is to ELECTRICITY magnets are wound with many turns of fine wire, and the terminals of the coil are connected directly with the brushes; hence the current arriving at the positive brush has two paths by which it may return to the negative, one around through the main circuit, the other through the field-magnet coils; the proportion of the current in each path is inversely as the resistance. Since but a small amount of the current is needed to energize the field, the coils are made of high resistance. If, in the last case, a few turns of the main circuit are placed on the field, they become compoundwound. (See Fig. 72.) $83. CONSTANT-CURRENT MACHINES. The load of a dynamo is usually added in one of two ways. In the first, the resistances are added in series, as in the case of arc lamps, and each lamp means so much resistance added to the main circuit. This would require that the machine should increase its potential as the load is added, or, what is the same thing, maintain a constant current. In the case of the series-wound machine, more resistance added to the main circuit would lessen the current, thus weakening the fields, when they should be made stronger. When a series-wound machine is used under these conditions, some mechanical device is used to regu⚫late the field by cutting turns into or out of the field-magnets. The shunt-wound machine, when an increased load means an increased resistance, would regulate, since, then, the greater the load or external resistance, the greater the current through the field-coils, but other disadvantages exclude the shunt in favor the series-wound with mechanically regulated fields for purposes requiring constant 1159 If a shunt-machine were used, a decrease of resistance in the external field would cause less current to go through the shunt, when there should be more. The current in the main circuit, however, is increased because its resistance has been decreased, but the increase is not sufficient to compensate. The drop or decrease in the shuntcurrent is not excessively large, and may be compensated by putting a few turns of the main circuit around the field-coils; it then becomes a compound-wound machine, and beautifully self-regulating for constant potential. (Fig. 72.) An 885. ALTERNATING-CURRENT DYNAMOS. alternating-current dynamo is one in which the current is periodically changing its direction, as in the case of the current resulting from the E. M. F. generated by the single coil of Fig. 61. The time taken for one complete alternation to and fro is called the period, and the number of complete alternations per second is called the frequency. The frequency used in practice varies from 50 to 150 per second. Since a two-pole machine gives but one alternation per revolution, alternators are multipolar, with 8 to 22 or more poles; the number of alternations per revolution of the armature is equal to the number of pairs of poles. By this means the desired number of alternations per second is obtained without increasing the speed of the armature beyond practical limits. N The arrangement of the armature-coils and field-magnet is shown diagrammatically in Fig. 73. The coils of the armature are turned to the front, in order that the method of connecting them may be seen. The two contact-rings at which the armature circuit terminate, are also turned to show the connection; the contactrings are two complete N rings insulated from the shaft, but revolving with it, the connection with the outside circuit being made with brushes, as usual. Since the poles of the field-magnet alternate north and south, the alternate coils of the armature are reversed as to the direction of the winding, so that the current induced by all poles is in the same direction at any one time; the current reverses each time the coils pass the point midway between the poles. The armature of an alternating-current dynamo S Fig. 73. |