y the act of the operator on plugging in and serving to
remove the line relay from the circuit whenever and as long as a plug is
inserted into any jack of the line. This seems to involve a considerable
expense in relays, but this, as has been stated, is warranted by the
greater simplicity in jacks which the use of the cut-off relay makes
possible. In addition to this expense of investment in the line and
cut-off relays, the amount of current required to hold up the cut-off
relays during conversations foots up to a considerable item of expense,
particularly as the system of supervisory signals is one in which the
supervisory lamp takes current not only while burning, but its circuit
takes even more current when the lamp is extinguished during the time of
a connection. For all of these reasons, and some other minor ones, it
was deemed expedient by the engineers of the Western Electric Company to
design a common-battery multiple switchboard for small and medium-sized
exchanges in which certain sacrifices might be made to the end of
accomplishing certain savings. The result has been a type of
switchboard, designated the No. 10, which may be found in a number of
Bell exchanges, it being considered particularly adaptable to
installations of from 500 to 3,000 lines. Although this board has been
subject to a good deal of adverse criticism, and although it seems
probable that even for the cheaper boards the No. 1 type with some of
the modifications just described will eventually supersede this No. 10
board, yet the present extent of use of the No. 10 board and the
instructive features which its type displays warrant its discussion
here.

_Circuits._ The circuits of this switchboard are shown in Fig. 349, this
indicating two-line circuits and a connecting cord circuit, together
with the auxiliary apparatus employed in connection with the operator's
telephone circuit, the pilot and night alarm circuits. The most
noticeable feature is that cut-off jacks are employed, the circuit of
the line normally extending through the sets of jack springs in the
multiple, and answering jacks to the line relay and battery on one side
of the line, and to ground on the other side. Obviously, the additional
complexity of the jack saves the use of a cut-off relay and the relay
equipment of each line consists, therefore, of but a single line relay,
which controls the lamp in an obvious manner.

[Illustration: Fig. 349. Western Electric No. 10 Board]