In such cases the arrangement shown in Fig. 141 is sometimes employed.  This shows two pairs of lines connected by the impedance-coil system with common terminals 1 and 2, between which ordinarily the common battery would be connected.  Instead of putting a battery between these terminals, however, at the local exchange, a condenser of large capacity is connected between them and from these terminals circuit wires 3 and 4 are led to a battery of suitable voltage at a distant central office.  The condenser in this case is used to afford a short-circuit path for the voice currents that leak from one side of one pair of lines to the other, through the impedance coils bridged across the line.  In this way the effect of the necessarily high resistance in the common leads 3 and 4, leading to the storage battery, is overcome and the tendency to cross-talk between the various pairs of connected lines is eliminated.  Frequently, instead of employing this arrangement, a storage battery of small capacity will be connected between the terminals 1 and 2, instead of the condenser, and these will be charged over the wires 3 and 4 from a source of current at a distant point.

A consideration of the various methods of supplying current from a common source to a number of lines will show that it is essential that the resistance of the battery itself be very low.  It is also necessary that the resistance and the impedance of the common leads from the battery to the point of distribution to the various pairs of lines be very low, in order that the voice currents which flow through them, by virtue of the conversations going on in the different pairs of lines, shall not produce any appreciable alteration in the difference of potential between the battery terminals.

CHAPTER XIV

THE TELEPHONE SET

We have considered what may be called the elemental parts of a complete telephone; that is, the receiver, transmitter, hook switch, battery, generator, call bell, condenser, and the various kinds of coils which go to make up the apparatus by which one is enabled to transmit and receive speech and signals.  We will now consider the grouping of these various elements into a complete working organization known as a telephone.

Before considering the various types it is well to state that the term telephone is often rather loosely used.  We sometimes hear the receiver proper called a telephone or a hand telephone.  Since this was the original speaking telephone, there is some reason for so calling the receiver.  The modern custom more often applies the term telephone to the complete organization of talking and signaling apparatus, together with the associated wiring and cabinet or standard on which it is mounted.  The name telephone set is perhaps to be preferred to the word telephone, since it tends to avoid misunderstanding as to exactly