h in the various coils--whether
non-inductive or inductive, or whether having one or several
windings--and in the condenser, a variety of tools by which he may
achieve a great many useful ends in his circuit work. Obviously, the
condenser affords a means for transmitting voice currents or
fluctuating currents, and for excluding steady currents. Likewise the
impedance coil affords a means for readily transmitting steady
currents but practically excluding voice currents or fluctuating
currents. By the use of these very simple devices it is possible to
sift out the voice currents from a circuit containing both steady and
fluctuating currents, or it is possible in the same manner to sift out
the steady currents and to leave the voice currents alone to traverse
the circuit.

Great use is made in the design of telephone circuits of the fact that
the electromagnets, which accomplish the useful mechanical results in
causing the movement of parts, possess the quality of impedance. Thus,
the magnets which operate various signaling relays at the central
office are often used also as impedance coils in portions of the
circuit through which it is desired to have only steady currents pass.
If, on the other hand, it is necessary to place a relay magnet, having
considerable impedance, directly in a talking circuit, the bad effects
of this on the voice currents may be eliminated by shunting this coil
with a condenser, or with a comparatively high non-inductive
resistance. The voice currents will flow around the high impedance of
the relay coil through the condenser or resistance, while the steady
currents, which are the ones which must be depended upon to operate
the relay, are still forced in whole or in part to pass through the
relay coil where they belong.

In a similar way the induction coil affords a means for keeping two
circuits completely isolated so far as the direct flow of current
between them is concerned, and yet of readily transmitting, by
electromagnetic induction, currents from one of these circuits to the
other. Here is a means of isolation so far as direct current is
concerned, with complete communication for alternating current.

CHAPTER XIII

CURRENT SUPPLY TO TRANSMITTERS

The methods by which current is supplied to the transmitter of a
telephone for energizing it, may be classified under two divisions:
first, those where the battery or other source of current is located
at the station with the transm