uit rather than a grounded circuit. The student should become
accustomed to the replacing of one of the line wires of a metallic
circuit by the earth, and to the method, employed in Figs. 125 and
126, of indicating a grounded circuit as distinguished from a metallic
circuit.

[Illustration: Fig. 126. Local-Battery Stations with Metallic Circuit]

In Fig. 127 is shown a slight modification of the circuit shown in
Fig. 126, which consists of connecting one end of the primary winding
to one end of the secondary winding of the induction coil, thus
linking together the primary circuit and the line circuit, a portion
of each of these circuits being common to a short piece of the local
wiring. There is no difference whatever in the action of the circuits
shown in Figs. 126 and 127, the latter being shown merely for the
purpose of bringing out this fact. It is very common, particularly in
local-battery circuits, to connect one end of the primary and the
secondary windings, as by doing so it is often possible to save a
contact point in the hook switch and also to simplify the wiring.

[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 pe