Wireless Telegraphy and Telephony

36 WIRELESS TELEGRAPHY of insulated wire, forming a primary and a secondary, wound, upon a rectangular core like that shown in Fig. 39B. The core is built up of sheets of iron called laminations, to reduce the heating and increase the efficiency of the machine. A. 8 Fig. 40.—Lines representing direct and intermittent direct currents. As noted above currents are only induced in a coil when the magnetic field is changing. The interrupter is em- ployed to rapidly “make” and “break” the circuit. Every time that the circuit is made the primary coil creates a field and every time it is broken it is destroyed. A direct current is a current which passes in one direction only. It may be represented by a straight line as A in Fig. 40. Its voltage is usually very constant and does not vary greatly. In the case of electric lighting circuits the normal voltage is usually no. If an interrupter is included in the circuit the current may be represented by a broken line, the spaces corresponding to the periods when the Fig. 41.—Diagram representing alternating current. current is “broken” and the lines to the periods it is flow- ing. The interrupter creates an intermittent direct cur- rent. An alternating current is one which reverses its direc- tion and passes first one way and then the other. It may