Wireless Telegraphy and Telephony

WIRELESS TELEGRAPHY 33 dred times as many turns of wire as the primary the induced voltage will be one hundred times the voltage of the original primary current. The purpose of the iron core is to concentrate the magnetic field and make the coil more efficient. Since currents are only induced in the secondary when the magnetic field is changing, an automatic device called an interrupter or sometimes a vibrator, is employed to rapidly turn the current flowing through the primary on and off. The interrupter consists Fig. 36.—Induction coil, primary and secondary. of a spring carrying a platinum point against which presses a second piece of platinum on the end of an adjustable thumbscrew. Platinum is necessary because the current of electricity would quickly oxidize and burn up any other material. The interrupter spring is placed near the end of the core so that the magnetism of the latter will draw it forward away from the thumbscrew and interrupt the current. As soon as the current ceases to flow the core loses its magnetism and the spring returns to its former position repeating the cycle very rapidly a large number of times per second. The interrupter is fitted with a con- denser shunted across its terminals to stop sparking at the platinum points and also to make the currents in the secondary more intense.