[Illustration:  Fig. 127.  Local-Battery Stations with Metallic Circuit]

The advantages to be gained by employing a local battery at each subscriber’s station associated with the transmitter in the primary circuit of an induction coil are attended by certain disadvantages from a commercial standpoint.  The primary battery is not an economical way to generate electric energy.  In all its commercial forms it involves the consumption of zinc and zinc is an expensive fuel.  The actual amount of current in watts required by a telephone is small, however, and this disadvantage due to the inexpensive method of generating current would not in itself be of great importance.  A more serious objection to the use of local batteries at subscribers’ stations appears when the subject is considered from the standpoint of maintenance.  Batteries, whether of the so-called “dry” or “wet” type, gradually deteriorate, even when not used, and in cases where the telephone is used many times a day the deterioration is comparatively rapid.  This makes necessary the occasional renewals of the batteries with the attendant expense for new batteries or new material, and of labor and transportation in visiting the station.  The labor item becomes more serious when the stations are scattered in a sparsely settled community, in which case the visiting of the stations, even for the performance of a task that would require but a few minutes’ time, may consume some hours on the part of the employes in getting there and back.

Common Battery. Advantages. It would be more economical if all of the current for the subscribers’ transmitters could be supplied from a single comparatively efficient generating source instead of from a multitude of inefficient small sources scattered throughout the community served by the exchange.  The advantage of such centralization lies not only in more economic generating means, but also in having the common source of current located at one place, where it may be cared for with a minimum amount of expense.  Such considerations have resulted in the so-called “common-battery system,” wherein the current for all the subscribers’ transmitters is furnished from a source located at the central office.

Where such a method of supplying current is practiced, the result has also been, in nearly all cases, the doing away with the subscriber’s magneto generators, relying on the central-office source of current to furnish the energy for enabling the subscriber to signal the operator.  Such systems, therefore, concentrate all of the sources of energy at the central office and for that reason they are frequently referred to as central-energy systems.