trical energy is
important. It not only dispenses with the necessity of running the
generators continuously, but it also affords a safeguard against
breakdowns which is one of its important uses.

The ability to carry the load of a central office directly on the
charging generator without the use of a storage battery is of no
importance except in an emergency which takes the storage battery wholly
out of service. Since the beginning of common-battery working such
emergencies have happened a negligible number of times. Far more
communities have lacked telephone service because of accidents beyond
human control than because of storage-battery failures.

In power plants serving large offices, the demand upon the storage
battery is great enough to require large plate areas in each cell. The
internal resistance, therefore, is small and considerable fluctuations
may exist in the charging current without their being heard in the
talking circuits. The amount of noise to be heard depends also on the
type of charging generator. Increasing the number of armature coils and
commutator segments increases the smoothness of the charging current.
The shape of the generator pole pieces is also a factor in securing such
smoothness.

If, with a given machine and storage battery, the talking circuits are
disturbed by the charging current, relief may be obtained by inserting a
large impedance in the charging circuit. This impedance requires to be
of low resistance, because whatever heat is developed in it is lost
energy. This means that the best conditions exist when the resistance is
low and the inductance large. These conditions are satisfied by using in
the impedance coil many turns of large wire and an ample iron core.

Dynamotors are not generally suitable for charging purposes. Not only is
the difficulty in regulating their output a disadvantage, but the fact
that the primary and secondary windings are so closely associated on the
armature core makes them carry into the charging current, not only the
commutator noises of the generator end, but of the motor end as well.

_Mercury-Arc Rectifiers._ In common-battery offices serving a few
hundred lines, and where the commercial supply is alternating current,
it is good practice to transform it into direct-battery charging current
by means of a mercury-arc rectifier. It is a device broadly similar to
the mercury-arc lamp produced by Peter Cooper Hewitt. It contains no
moving parts and