inertia, and opposes any attempt to start or stop it. This inertialike effect of a coil upon itself is called self-induction.

Let a few dry cells be inserted into a circuit containing a coil of a large number of turns of wire, the circuit being closed at some point by touching two bare copper wires together. Holding the bare wire in the fingers, break the circuit between the hands and observe the shock due to the current which the E.M.F. of self-induction sends through your body. Without the coil in circuit you will obtain no such shock, though the current stopped when you break the circuit will be many times larger.

366. The induction coil. The induction coil, as usually made (Fig. 323), consists of a soft iron core C' composed of a bundle of soft iron wires; a primary coil p wrapped around this core and consisting of, say, 200 turns of coarse copper wire

(1)

dH

t't

(2)

Condenser

FIG. 323. Induction coil

(for example, No. 16), which is connected into the circuit of a battery through the contact point at the end of the screw d; a secondary coil & surrounding the primary in the manner indicated in the diagram and consisting generally of between 30,000 and 1,000,000 turns of No. 36 copper wire, the terminals of which are the points t and t'; and a hammer b, or other automatic arrangement for making and breaking the circuit of the primary. (See ignition system opposite p. 199.)

Let the hammer b be held away from the opposite contact point by means of the finger, then touched to this point, then pulled quickly away. A spark will be found to pass between t and t' at break only - never at make. This is because, on account of the opposing influence at make of selfinduction in the primary, the magnetic field about the primary rises

very gradually to its full strength, and hence its lines pass into the secondary coil comparatively slowly. At break, however, by separating the contact points very quickly we can make the current in the primary fall to zero in an exceedingly short time, perhaps not more than .00001 second; that is, we can make all of its lines pass out of the coil in this time. Hence the rate at which lines thread through or cut the secondary is perhaps 10,000 times as great at break as at make, and therefore the E.M.F. is also something like 10,000 times as great. In the normal use of the coil the circuit of the primary is automatically made and broken at b by means of the magnet and the spring r, precisely as in the case of the electric bell. Let the student analyze this part of the coil for himself. The condenser shown in the diagram, with its two sets of plates connected to the conductors on either side of the spark gap between r and d, is not an essential part of a coil, but when it is introduced it is found that the length of the spark which can be sent across between t and is considerably increased. The reason is as follows: When the circuit is broken at b, the inertia (that is, the self-induction) of the primary current tends to make a spark jump across from d to b; and if this happens, the current continues to flow through this spark (or arc) until the terminals have become separated through a considerable distance. This makes the current die down gradually instead of suddenly, as it ought to do to produce a high E.M.F; but when a condenser is inserted, as soon as b begins to leave d the current begins to flow into the condenser, and this gives the hammer time to get so far away from d that an arc cannot be formed. This means a sudden break and a high E. M. F. Since a spark passes between t and only at break, it must always pass in the same direction. Coils which give 24-inch sparks (perhaps 500,000 volts) are not uncommon. Such coils usually have hundreds of miles of wire upon their secondaries.

FIG. 324.

Core of

insulated iron wire

367. Laminated cores; Foucault currents. The core of an induction coil should always be made of a bundle of soft-iron wires insulated from one another by means of shellac or varnish (see Fig. 324); for whenever a current is started or stopped in the primary p of a coil furnished with a solid iron core (see Fig. 325), the change in the magnetic field of the primary induces a current in the

conducting core C, for the same reason that it induces one in the secondary s. This current flows around the body of the core in the same direction as the induced current in the secondary, that is, in the direction of the arrows. The only effect of these so-called eddy or Foucault currents is to heat the core. This is obviously a waste of energy. If we can prevent the appearance

of these currents, all of the energy which they would waste in heating the core may be made to appear in the current of the secondary. The core is therefore built of varnished iron wires, which run parallel to the axis of the coil, that is, perpendicular to the direction in which the currents would be induced. The induced E.M.F. therefore finds no closed circuits in which to set up a current (Fig. 324). It is for the same reason that the iron cores of dynamo and motor armatures, instead of being solid, consist of iron disks placed side by side, as shown in Fig. 326, and insulated from one another by films of oxide. A core of this kind is called a laminated core. It will be seen that in all such cores the spaces or slots between the laminæ must run at right angles to the direction of the induced E.M.F., that is, perpendicular to the conductors upon the core.

FIG. 326. Laminated drum-armature core with commutator, showing one coil wound on the core

B

R

368. The transformer. The commercial transformer is a modified form of the induction coil. The chief difference is that the core R (Fig. 327), instead of being straight, is bent into the form of a ring or is given some other shape such that the magnetic lines of force have a continuous iron path instead of being obliged to push out into the air, as in the induction coil. Furthermore, it is always an alternating instead of an intermittent current which is sent through the primary A. Sending such a current through A is equivalent to first magnetizing the core in one direction, then demagnetizing it, then magnetizing it in the opposite direction, etc. The result of

Ꭱ

FIG. 327. Diagram of transformer

these changes in the magnetism of the core is of course an induced alternating current in the secondary B.

Main Conductor

Main Conductor

[Alternator

369. The use of the transformer. The use of the transformer is to convert an alternating current from one voltage to another which, for some reason, is found to be more convenient. For example, in electric lighting where an alternating current is used, the E.M.F. generated by the dynamo is usually either 1100 or 2200 volts, a voltage too high to be introduced safely into private houses. Hence transformers are connected across the main conductors in the manner shown in Fig. 328. The current which passes into the houses to supply the lamps does not come directly from the dynamo. It is an induced current generated in the transformer.

Lamps

FIG. 328. Alternating-current lighting circuit with transformers

Through the use of small transformers the voltage of the current of the house lighting circuit is further reduced and made available for the ringing of doorbells.

370. Pressure in primary and secondary. If there are a few turns in the primary and a large number in the secondary, the transformer is called a step-up transformer, because the P.D, produced at the terminals of the secondary is greater than that applied at the terminals of the primary. In electric lighting, transformers are mostly of the step-down type; that is, a high P.D. (say, 2200 volts) is applied at the terminal of the primary, and a lower P.D. (say, 110 volts) is obtained at the terminals of the secondary. In such a transformer the primary will have twenty times as many turns as the secondary. In general, the ratio between the voltages at the terminals of the primary and secondary is the ratio of the number of turns of wire upon the two.

371. Efficiency of the transformer. In a perfect transformer the efficiency would be unity. This means that the electrical power, or watts, put into the primary (that is, the volts applied to its terminals times the amperes flowing through it) would be exactly equal to the power, or watts, taken out in the secondary (that is, the volts generated in it times the strength of the induced current); and, in fact, in actual transformers the latter product is often more than 97% of the former (that is, there is less than 3% loss of energy in the transformation). This lost energy appears as heat in the transformer. This transfer, which goes on in a big transformer, of huge quantities of power from one circuit to another entirely independent circuit, without noise or motion of any sort and almost without loss, is one of the most wonderful phenomena of modern industrial life.

372. Commercial transformers. Fig. 329 illustrates a common type of transformer used in electric lighting. The core is built up of sheet-iron laminæ about millimeter thick. Fig. 330 shows a section of the same

transformer. The closed magnetic circuit of the core is indicated by the dotted lines. The primaries and the secondaries are indicated by the letters P and S. Fig. 331 is the case in which the transformer is placed. Such cases may be seen attached to poles outside of houses wherever alternating currents are used for electric lighting (Fig. 332).

373. Electrical transmission of power. Since the rate of production of electrical energy by a dynamo is the product of the E.M.F. generated by the current furnished, it is evident that in order to transmit from