e of an electromagnet which receives
current impulses from the line circuit. There is thus one of these
driving magnets at each station, each bridged across the line so that
when a single impulse of current is sent out from the central office
all of the ratchet wheels will be moved one step. Another impulse will
move all of the ratchet wheels another step, and so on throughout any
desired number of impulses. The ratchet wheels, therefore, are all
stepped in unison.

Let us further conceive that all of these ratchet wheels are provided
with a notch or a hole or a projection, alike in all respects at all
stations save in the position which this notch or hole or projection
occupies on the wheel. The thing to get clear in this part of the
conception is that all of these notches, holes, or projections are
alike on all of the wheels, but they occupy a different position on
the wheel for each one of the stations.

Consider further that the bell circuit at each of the stations is
normally open, but that in each case it is adapted to be closed when
the notch, hole, or projection is brought to a certain point by the
revolution of the wheel.

Let us conceive further that this distinguishing notch, hole, or
projection is so arranged on the wheel of the first station as to
close the bell circuit when one impulse has been sent, that that on
the second station will close the bell circuit after the second
impulse has been sent, and so on throughout the entire number of
stations. It will, therefore, be apparent that the bell circuits at
the various stations will, as the wheels are rotated in unison, be
closed one after the other. In order to call a given station,
therefore, it is only necessary to rotate all of the wheels in unison,
by sending out the proper stepping impulses until they all occupy such
a position that the one at the desired station is in such position as
to close the bell circuit at that station. Since all of the notches,
holes, or projections are arranged to close the bell circuits at their
respective stations at different times, it follows that when the bell
circuit at the desired station is closed those at all of the other
stations will be open. If, therefore, after the proper number of
stepping impulses has been sent to the line to close the bell circuit
of the desired station, ringing current be applied to the line, it is
obvious that the bell of that one station will be rung to the
exclusion of all others. It