ber's station, and at the
right the central-office apparatus is indicated so far as equipment of a
single line is concerned. In this simplified diagram no attempt has been
made to show the relative positions of the various parts, these having
been grouped in this figure in such a way as to give as clear and simple
an idea as possible of the circuit arrangements. It is seen at a glance
that this is a branch terminal board, the three contacts of each jack
being connected by separate taps or legs to three wires running
throughout the length of the board, these three wires being individual
to the jacks of one line. On this account this line circuit is commonly
referred to as a three-wire circuit. By the same considerations it will
be seen that the switchboard line circuit of the branch-terminal
multiple magneto system, shown in Fig. 338, would be called a four-wire
circuit. It will be shown later that other multiple switchboards in wide
use have a still further reduction in the number of wires running
through the jacks, or through the multiple as it is called, such being
referred to as two-wire switchboards.

The two limbs of the line which extend from the subscriber's circuit,
beside being connected by taps to the tip and sleeve contacts of the
jack respectively, connect with the two back contacts of a cut-off
relay, and when this relay is in its normal or unenergized condition,
these two limbs of the line are continued through the windings of the
line relay and thence one to the ungrounded or negative side of the
common-battery and the other to the grounded side. The subscriber's
station circuit being normally open, no current flows through the line,
but when the subscriber removes his receiver for the purpose of making a
call the line circuit is completed and current flows through the coil of
the line relay, thus energizing that relay and causing it to complete
the circuit of the line lamp. The cut-off relay plays no part in the
operation of the subscriber's calling, but merely leaves the circuit of
the line connected through to the calling relay and battery. The coil of
the cut-off relay is connected to ground on one side and on the other
side to the third wire running through the switchboard multiple and
which is tapped off to each of the test rings on the jacks. As will be
shown later, when the operator plugs into the jack of a line, such a
connection is established that the test ring of that jack will be
connected to