Wireless Telegraphy and Telephony

8 WIRELESS TELEGRAPHY In the illustration the current from a battery is led into the primary of an induction coil. The primary is simply a coil consisting of a few turns of wire, which induces a high voltage in a second coil consisting of a larger number of turns, and called the secondary. The terminals of the secondary are led to a spark gap—an arrangement com- posed of two polished brass balls, separated by a small air space. One of the balls, in turn, is connected to a metal plate buried in the earth, and the other to a network of wires suspended high in the air and insulated from all sur- rounding objects. As noted above, a Leyden jar consists of two metallic coatings, separated by a wall of glass. The purpose of the coatings is to form a conductor and carry an electric charge. A Leyden jar possesses a characteristic called, in electricity, capacity. Any two conductors separated by an insulating medium possess “capacity” and all the proper- ties of a Leyden jar or condenser. The waves generated by a Leyden jar would be somewhat weak and confined to its own immediate neighborhood, so recourse is had to the aerial and ground, in order to in- crease the area over which the oscillations exert their in- fluence in setting up the electric waves. The aerial system corresponds to one coating of the Leyden jar, and the ground to the other. The insulating medium in be- tween, corresponding to the glass, or dielectric, is the at- mosphere. When the key connected to the induction coil is pressed, the battery current flows through the primary and induces a high voltage current in the secondary, which charges the aerial and ground exactly as the static machine charges the two coatings of the Leyden jar. A spark then leaps across the spark gap and the current surges back and forth through the aerial, generating “high frequency oscillations” which,