NOTE.  In this chapter the central-energy or common-battery system will be considered only in so far as the supply of current for energizing the subscribers’ transmitters is concerned, the discussion of the action of signaling being reserved for subsequent chapters.

Series Battery. If but a single pair of lines had to be considered, the arrangement shown in Fig. 128 might be employed.  In this the battery is located at the central office and placed in series with the two grounded lines leading from the central office to the two subscribers’ stations.  The voltage of this battery is made sufficient to furnish the required current over the resistance of the entire line circuit with its included instruments.  Obviously, changes in resistance in the transmitter at Station A will affect the flow of current in the entire line and the fluctuations resulting from the vibration of the transmitter diaphragm will, therefore, reproduce these sounds in the receiver at Station B, as well as in that at Station A.

[Illustration:  Fig. 128.  Battery in Series with Two Lines]

An exactly similar arrangement applied to a metallic circuit is shown in Fig. 129.  In thus placing the battery in series in the circuit between the two stations, as shown in Figs. 128 and 129, it is obvious that the transmitter at each station is compelled to vary the resistance of the entire circuit comprising the two lines in series, in order to affect the receiver at distant stations.  This is in effect making the transmitter circuit twice as long as is necessary, as will be shown in the subsequent systems considered.  Furthermore, the placing of the battery in series in the circuit of the two combined lines does not lend itself readily to the supply of current from a common source to more than a single pair of lines.

[Illustration:  Fig. 129.  Battery in Series with Two Lines]

Series Substation Circuit. The arrangement at the substations—­consisting in placing the transmitter and the receiver in series in the line circuit, as shown in Figs. 128 and 129—­is the simplest possible one, and has been used to a considerable extent, but it has been subject to the serious objection, where receivers having permanent magnets were used, of making it necessary to so connect the receiver in the line circuit that the steady current from the battery would not set up a magnetization in the cores of the receiver in such a direction as to neutralize or oppose the magnetization of the permanent magnets.  As long as the current flowed through the receiver coils in such a direction as to supplement the magnetization of the permanent magnets, no harm was usually done, but when the current flowed through the receiver coils in such a way as to neutralize or oppose the magnetizing force of the permanent magnets, the action of the receiver was greatly interfered with.  As a result, it was necessary to always connect the receivers in the line circuit in a certain way, and this operation was called poling.