on of consumption in one circuit leading from it causing slight
variation in the currents of other circuits leading from it. If this
were not so, cross-talk would exist between the telephones of the
operators' positions connected to the same battery. This regulating
quality enables the multiple feeding of telephone circuits to be carried
further than the mere supplying of operators' sets and is the quality
which makes possible the successful use of a storage battery as the
single source of transmitter current for common-battery central-office
equipment.

In furnishing a plurality of operators' transmitters from a common
battery, the importance of low resistance and inductance in the portion
of the path that is common to all of the circuits must not be
overlooked. Not only is a battery of extremely low resistance required,
but also conductors leading from it that are common to two or more of
the circuits should be of very low resistance and consequently large in
cross-section and as short as possible. In common-battery offices there
is obviously no need of employing a separate battery for the operators'
transmitters, since they may readily be supplied from the common storage
battery which supplies direct current to the subscribers' lines.

=Ringing-Current Supply.= _Magneto Generators._ As a central-office
equipment is required to ring many subscribers' bells, only the small
ones find it convenient to ring them by means of hand-operated magneto
generators. Small magneto switchboards are usually equipped so that each
operator is provided with a hand-generator, but even where such is the
case some source of ringing current not manually operated is desirable.
In larger switchboards the hand generators are entirely dispensed with.

The magneto generator may be driven by a belt from any convenient
constantly moving pulley, and the early telephone exchanges were often
equipped with such generators having better bearings and more current
capacity than those in magneto telephones. These were adapted to be run
constantly from some source of power, delivering ringing current to the
operators' keyboards at from 16 to 20 cycles per second.

_Pole Changers._ Vibrating pole changers were also used in the early
exchanges, but passed out of use, partly because of poor design, but
more because of the absence of good forms of primary batteries for
vibrating them and for furnishing the direct currents to be transformed
into alternating