and this vertical plane is
made to oscillate back and forth by an oscillating shaft so as always
_to point the plungers toward a vertical row of trunk contacts that
represent a trunk that is not in use at the time_. The to-and-fro
movement of this oscillating shaft, called the _master bar_, is
controlled by a master switch and the function of this master switch is
always to keep the plungers pointed toward the row of contacts of an
idle trunk. The thrusting movement of the individual plungers into the
contact bank is controlled by magnets individual to the line and under
control of the subscriber in initiating a call. As soon as the plunger
of a line has been thus thrust into the contact bank so as to connect
the terminals of that line with a given trunk, the plunger is no longer
controlled by the master bar and remains stationary. The master bar then
at once moves all of the other plungers that are not in use so that they
will point to the terminals of another trunk that is not in use. The
plungers of all the line switches in a group of twenty-five are,
therefore, subject to the oscillating movements of the master bar when
the line is not connected to a first selector trunk. As soon as a call
is originated on a line, the corresponding plunger is forced into the
bank and is held stationary in maintaining the connection to a first
selector trunk, and all of the other plungers not so engaged, move on so
as to be ready to engage another idle trunk.

_Trunk Ratio._ The assignment of ten trunks to twenty-five lines would
be a greater ratio of trunks than ordinary traffic conditions require.
This ratio of trunks to lines is, however, readily varied by multipling
the trunk contacts of several twenty-five line groups together. Thus,
ten trunks may be made available to one hundred subscribers' lines by
multipling the trunks of four twenty-five line switch groups together.
In this case the four master bars corresponding to the four groups of
twenty-five line switches are all mechanically connected together so as
to move in unison under the control of a single master switch. If more
than ten and less than twenty-one trunks are assigned to one hundred
lines, then each set of ten trunks is multipled to the trunk contacts of
fifty line switches, the two master bars of these switches being
connected together and controlled by a common master switch.

_Structure of Line Switch._ The details of the parts of a line switch
that are indi