None of these objections are present, however, when recourse is had to Hertzian, or electromagnetic waves. Wireless transmission of speech has therefore followed in the wake of wireless telegraphy, and the methods and apparatus employed are very similar.
Some who have followed the text closely might reasonably ask why it would not be possible to establish wireless telephony by simply connecting a telephone transmitter in some manner to an ordinary wireless telegraph, and by directing speech into the latter, vary the strength of the oscillations emitted.
Such a system, at first thought, seems very plausible, and many experimenters have devised countless methods trying to attain this result, only to meet with ultimate failure. The reason is very simple.
Suppose that an induction coil, having a high-speed interruptor, and therefore able to produce a very rapid stream of sparks at the gap, is connected to the aerial and ground in the usual manner and a telephone transmitter placed in series with the ground wire. When the coil is set in operation the sparks jump across the gap, each spark setting up a train of oscillations. If speech is conveyed into the transmitter, the resistance in the path of the oscillations will be varied and correspondingly also the strength of the waves emitted. The sounds will be reproduced to a certain extent by the receptor. Whistling, certain musical tones, and words containing many vowels are sometimes heard in the receptor, with sufficient distinctness to be recognizable. The voice cannot, however, be heard at all times, and the system is of no real value other than an interesting experiment.
FIG. 139.—A "logical" form of wireless telephone which is impracticable.
FIG. 140.—De Forest wireless telephone equipment.
The reason is very simple and readily explained. For the sake of clearness we will suppose that the speed of the interruptor attached to the coil is 100 per second. It will therefore produce 100 sparks per second at the spark gap if the electrodes are close together. The passage of the sparks is not continuous, each one only occupying a very small space of time. The pause between each is very distinct, although it could not be detected with the naked eye. The ten straight lines in Fig. 141 represent ten sparks which cover a period of one-tenth of a second, since they pass at the rate of 100 per second. Each spark produces a train of oscillations, which surge back and forth in the aerial, rapidly dying out, however, or becoming damped in the manner already explained.
