operates at high efficiency without introducing noises
into the telephone lines. It requires little care and has good length of
life.

[Illustration: Fig. 415. Mercury-Arc Rectifier Circuits]

The circuit of a mercury-arc rectifier charging outfit is shown in Fig.
415. The mercury-arc rectifier proper consists of a glass bulb
containing vacuum and a small amount of mercury. When its terminals are
connected, as indicated--the two anodes across an alternating-current
source and the cathode with a circuit that is to be supplied with direct
current--this device has the peculiarity of action that current will
flow alternately from the two anodes always to the cathode and never
from it. The cathode, therefore, becomes a source of positive potential
and, as such, is used in charging the storage battery through the series
reactance coil and the compensating reactances, as indicated. The line
transformer shown at the upper portion of Fig. 415, is the one for
converting the high-potential alternating current to the comparatively
low-potential current required for the action of the rectifier. The
transformer below this has a one-to-one ratio, and is called the
insulating transformer. Its purpose is to safeguard the telephone
apparatus and circuits against abnormal potentials from the line, and
also to prevent the ground, which is commonly placed on the neutral wire
of transformers on commercial lighting circuits, from interfering with
the ground that is commonly placed on the positive pole of the
central-office battery.

=Provision Against Breakdown.= In order to provide against breakdown of
service, a well-designed telephone power plant should have available
more than one primary source of power and more than one charging unit
and ringing unit.

_Duplicate Primary Sources._ In large cities where the commercial power
service is highly developed and a breakdown of the generating station is
practically impossible, it is customary to depend on that service alone.
In order to insure against loss of power due to an accident to portions
of the distributing system, it is the common custom to run two entirely
separate power leads into the office, coming, if possible, from
different parts of the system so that a breakdown on one section will
not deprive the telephone exchange of primary power. In smaller places
where the commercial service is not so reliable, it is usual to provide,
in addition to the commercial electric-power service,