Wireless telegraphy, as it is practiced to-day, is based upon the fact that a system of wires or circuits, through which high frequency oscillations are surging, becomes a source of electromagnetic waves. Various methods have been devised for making the system more efficient and capable of giving better results with a given amount of power.
FIG. 7.—The simplest practical transmitter that it is possible to devise for the purpose of sending messages.
Fig. 7 is a diagram showing the simplest practical transmitter that it is possible to devise for the purpose of sending messages a sufficient distance to be of any value.
It would be impractical to use a static electric machine for wireless transmission, and so an induction coil or transformer is employed. These latter instruments are for the purpose of raising electric currents of a comparatively low voltage to the high potential, where they have the power of generating high frequency oscillations.
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 composed 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 surrounding 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 properties 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 increase the area over which the oscillations exert their influence 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 between, corresponding to the glass, or dielectric, is the atmosphere.
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, in turn, set up a state of strain in the surrounding ether, and cause the waves to travel out from the system.
