practice the voice currents be relayed in both directions, and
further, that the relay actually augment the energy which passes
through it; that is, that it will send on a more powerful current than
it receives. Most of the devices so far invented fail in one or the
other of these particulars. Several ways have been shown of assembling
repeating devices which will talk both ways, but not many assembling
repeating devices have been shown that will talk both ways and augment
in both directions.

[Illustration: Fig. 36. Shreeve Repeater and Circuit]

Practical repeaters have been produced, however, and at least one type
is in daily successful use. It is not conclusively shown even of it
that it augments in the same degree all of the voice waves which reach
it, or even that it augments some of them at all. Its action, however,
is distinctly an improvement in commercial practice. It is the
invention of Mr. Herbert E. Shreeve and is shown in Fig. 39.
Primarily it consists of a telephone receiver, of a particular type
devised by Gundlach, associated with a granular carbon transmitter
button. It is further associated with an arrangement of induction
coils or repeating coils, the object of these being to accomplish the
two-way action, that is, of speaking in both directions and of
preventing reactive interference between the receiving and
transmitting elements. The battery _1_ energizes the field of the
receiving element; the received line current varies that field; the
resulting motion varies the resistance of the carbon button and
transforms current from battery _2_ into a new alternating line
current.

By reactive interference is meant action whereby the transmitter