Inductance is the property of a circuit by virtue of which lines of force are developed around it. Circuits containing a certain amount of inductance, capacity and resistance tend to oscillate electrically at a certain frequency.

FIG. 26. Electric Waves.

The electromagnetic waves thrown off by the aerial system follow the contour of the earth and so may cross mountains or travel anywhere. The waves emitted by the ordinary wireless station, making use of an aerial and a ground are half waves terminating in the earth as shown in the illustration. In passing over the earth they are accompanied by ground currents which waste a certain amount of their energy in overcoming ohmic resistance and so reduce the intensity of the waves. For this reason propagation is always the best over water or moist earth whose resistance is low.

A further peculiar weakening of the waves due to the absorbtion taking place in the air during sunlight. The difference between the signals in the day and their strength at night is very marked, being much stronger in the later case.

LESSON TWELVE. PRINCIPLES OF WAVE TELEGRAPHY.

Wireless Telegraphy as practiced to-day is merely a method of setting up electromagnetic waves in the ether and then detecting their existence at a distant point. It may be divided into four distinct and individual operations, namely:

  1. The generation of electrical oscillations.
  2. The transformation of electrical oscillations into electrical waves.
  3. The transformation of electrical waves into electrical oscillations.
  4. The detection of the electrical oscillations.

We have already learned how electrical oscillations may be generated by the discharge of a Leyden jar or a condenser. In order to perform the first two operations named above, it is therefore merely necessary to arrange a condenser in such a way that it is most effective.

The induction coil or transformer is employed to charge the condenser because the currents of these instruments are much more powerful than those of a static electric machine. The induction coil is connected to a set of batteries and a key so that the periods during which the current is on and off may be controlled at will by the pressure of the fingers.