Means for Assorting Currents.  In conclusion, it is obvious that the telephone engineer has within his reach 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 transmitter which it supplies; and second, those where the battery or other source of current is located at a distant point from the transmitter, the battery in such cases serving as a common source of current for the supply of transmitters at a number of stations.