element, in emitting a wave, affects its own controlling receiver
element, thus setting up an action similar to that which occurs when
the receiver of a telephone is held close to its transmitter and
humming or singing ensues. No repeater is successful unless it is free
from this reactive interference.

[Illustration: Fig. 37. Mercury-Arc Telephone Relay]

Enough has been accomplished by practical tests of the Shreeve device
and others like it to show that the search for a method of relaying
telephone voice currents is not looking for a pot of gold at the end
of the rainbow. The most remarkable truth established by the success
of repeaters of the Shreeve type is that a device embodying so large
inertia of moving parts can succeed at all. If this mean anything, it
is that a device in which inertia is absolutely eliminated might do
very much better. Many of the methods already proposed by inventors
attack the problem in this way and one of the most recent and most
promising ways is that of Mr. J.B. Taylor, the circuit of whose
telephone-relay patent is shown in Fig. 37. In it, _1_ is an
electromagnet energized by voice currents; its varying field varies an
arc between the electrodes _2-2_ and _3_ in a vacuum tube. These
fluctuations are transformed into line currents by the coil _4_.




CHAPTER V

TRANSMITTERS