=307. The Induction Coil.=--Practically all electric currents are produced either by voltaic cells or by dynamos. It is frequently found, however, that it is desirable to change the E.M.F. of the current used, either for purposes of _effectiveness_, _convenience_, _or economy_. The _induction coil_ and the _transformer_, devices for changing the E.M.F. of electric currents, are therefore in common use. _The induction coil_ (see Fig. 302) consists of a _primary_ coil of coarse wire _P_ (Fig. 303) wound upon a core of soft iron wire, and a _secondary_ coil, _S_, of several thousand turns of fine wire. In circuit with the primary coil is a battery, _B_, and a current interrupter, _K_, which works like the interrupter upon an electric bell. The ends of the secondary coil are brought to binding posts or spark points as at _D_. [Illustration: FIG. 302.--An induction coil.] The current from the battery flows through the primary coil magnetizing the iron core. The magnetism in the core attracts the soft-iron end of the interrupter, drawing the latter over and breaking the circuit at the screw contact, _K_. This abruptly stops the current and at once the core loses its magnetism. The spring support of the interrupter now draws the latter back to the contact, _T_, again completing the circuit. The whole operation is repeated, the interrupter vibrating rapidly continually opening and closing the circuit. [Illustration: FIG. 303.--Diagram showing the parts of an induction coil.] =308. The Production of Induced Currents in the Secondary Coil.=--When the current flows through the _primary_ it sets up a magnetic field in the _core_. When the current is interrupted, the field disappears. The increase and decrease in the field of the core induces an E.M.F. in the secondary coil, in accordance with the first law of electromagnetic induction. The E.M.F. produced depends upon (a) the number of turns in the secondary, (b) the strength of the magnetic field and (c) the rate of change of the field. The rate of change in the field is more rapid at the break than at the make. When the circuit is closed it takes perhaps 1/10 of a second for the current to build up to its full strength while at a break the current stops in perhaps 0.00001 of a second, so that the induced E.M.F. is perhaps 10,000 times as great at "break" as at make. To increase the suddenness of the "make" and "break," a condenser is often connected in the primary circuit, in parallel, with the interrupter. (See Fig. 303, _C._) This condenser provides a place to hold the rush of current at the instant that the interrupter breaks the circuit. This stored up charge reinforces the current at the make producing a much more sudden change in the magnetic field with a corresponding increase in the E.M.F. The induced currents from induction coils are sometimes called _faradic currents_ in honor of Faraday who discovered electromagnetic induction. They are used to operate sparking