FIG. 27. Diagram of Wireless Transmitter.
The secondary of the coil is connected to a battery of Leyden jars or a condenser. The fact was mentioned above that a certain amount of inductance in the circuit is necessary for the production of electrical oscillations This is furnished, or at least the greater part, by a device called a helix which consists of a coil of heavy wire wound around a suitable framework.
The spark discharge takes place across a device called a spark gap.
When the key is pressed, the high potential currents of the induction coil charge the Leyden jar or condenser and cause it to discharge through the helix and across the spark gap. High frequency oscillations are immediately created in this part of the circuit. The spark gap, condenser and that part of the helix included, constitute the closed circuit. The electromagnetic waves thrown off by such an oscillatory system would not be very far reaching in their effects because the disturbances would be confined to the immediate neighborhood of the apparatus, so recourse is had to the aerial and ground. The aerial consists of a network of wires elevated high in the air. The ground or earth connection is simply a large metal plate buried in moist earth or thrown into the sea. By connecting the aerial and ground to the helix in the manner shown in Fig. 27, the high frequency currents are caused to surge up and down the aerial system into the ground and create very powerful electromagnetic waves which possess the power of exciting electrical oscillations in another aerial even though it may be located many miles away.
The existence of these oscillations is made known to the receiving operator by a device known as a detector, described fully in one of the following lessons.
LESSON THIRTEEN. THE AERIAL.
The aerial system or antenna might be termed the mouth and ear of the wireless station, for it is this huge network of wires stretching high into the air that emits or intercepts the electromagnetic waves upon which such systems of communication depend.
FIG. 28. General Types of Aerials