an independent
source of power in the form of a gas or steam engine. This may be run as
a regular source, the commercial service being employed as an emergency
or _vice versa_, as economy may dictate. In providing a gas engine for
driving charging dynamos, it is important to obtain one having as good
regulation as possible, in order to obtain a charging current of
practically constant voltage.

_Duplicate Charging Machines._ The storage batteries of telephone
exchanges are usually provided of sufficient capacity to supply the
direct-current needs of the office for twenty-four hours after a full
charge has been given them. This in itself is a strong safeguard against
breakdown. In addition to this the charging machines should be in
duplicate, so that a burnt-out armature or other damage to one of the
charging units will not disable the plant.

_Duplicate Ringing Machines._ It is equally important that the ringing
machines, whether of the rotary or vibrating type, be in duplicate. For
large exchanges the ringing machines are usually dynamos, and it is not
unusual to have one of these driven from the commercial power mains and
the other from the storage battery. With this arrangement complete
failure of all sources of primary power would still leave the exchange
operative as long as sufficient charge remains in the storage battery.

_Capacity of Power Units._ In designing telephone switchboards it is the
common practice to so design the frameworks that the space for multiple
jacks is in excess of that required for the original installation. In a
like manner, the power plant is also designed with a view of being
readily increased in capacity to an amount sufficient to provide current
for the ultimate number of subscribers' lines for which the switchboard
is designed. The motor generators, or whatever means are provided for
charging the storage batteries, are usually installed of sufficient size
to care for the ultimate requirements of the office. The ringing
machines are also provided for the ultimate equipment. However, in the
case of the storage battery, it is common practice to provide the
battery tanks of sufficient size to care for the ultimate capacity,
while the plates are installed for a capacity only slightly in excess of
that required for the original installation. As the equipment of
subscribers' lines is increased, additional plates may, therefore, be
added to the cells without replacing the storage battery a