[Illustration:  MAIN OFFICE, KEYSTONE TELEPHONE COMPANY, PHILADELPHIA, PA.]

[Illustration:  Fig. 138.  Common Source for Many Lines]

[Illustration:  Fig. 139.  Common Source for Many Lines]

Retardation Coil—­Single Battery:—­In Fig. 139 is shown in similar manner the current supply from a single battery to four different pairs of lines, the battery being associated with the lines by the combined impedance coil and condenser method, which was specifically dealt with in connection with Fig. 133.  The reasons why there will be no interference between the conversations carried on in the various pairs of connected lines in this case are the same as those just considered in connection with the system shown in Fig. 138.  The impedance coils in this case serve to keep the telephone currents confined to their respective pairs of lines in which they originate, and this same consideration applies to the system of Fig. 138, for each of the separate repeating-coil windings of Fig. 138 is in itself an impedance coil with respect to such currents as might leak away from one pair of lines on to another.

Retardation Coil—­Double Battery:—­The arrangement of feeding a number of pairs of lines according to the Kellogg two-battery system is indicated in Fig. 140, which needs no further explanation in view of the description of the preceding figures.  It is interesting to note in this case that the left-hand battery serves only the left-hand lines and the right-hand battery only the right-hand lines.  As this is worked out in practice, the left-hand battery is always connected to those lines which originate a call and the right-hand battery always to those lines that are called for.  The energy supplied to a calling line is always, therefore, from a different source than that which supplies a called line.

[Illustration:  Fig. 140.  Two Sources for Many Lines]

[Illustration:  Fig. 141.  Current Supply from Distant Point]

Current Supply from Distant Point. Sometimes it is convenient to supply current to a group of lines centering at a certain point from a source of current located at a distant point.  This is often the case in the so-called private branch exchange, where a given business house or other institution is provided with its own switchboard for interconnecting the lines leading to the various telephones of that concern or institution among themselves, and also for connecting them with lines leading to the city exchange.  It is not always easy or convenient to maintain at such private switchboards a separate battery for supplying the current needed by the local exchange.