and rotary motion; a pawl and ratchet mechanism actuated by
a magnet for moving the shaft vertically a step at a time; another pawl
and ratchet mechanism actuated by another magnet for rotating the shaft
a step at a time; an arm carrying wiper contacts on its outer end,
mounted on and moving with the shaft; and a bank of contacts arranged on
the inner surface of a section of a cylinder adapted to be engaged by
the wiper contacts on this movable arm.

These various elements are indicated in the merest outline and with much
distortion in Fig. 380, which is intended to illustrate the principles
of operation rather than the details as they actually are in the system.
In the upper left-hand corner of this figure, the magnet shown will, if
energized by impulses of current, attract and release its armature and,
in doing so, cause the pawl controlled by this magnet to move the
vertical shaft of the switch up a step at a time, as many steps as
there are impulses of current. The vertical movement of this shaft will
carry the wiper arm, attached to the lower end of the shaft, up the same
number of steps and, in doing so, will bring the contacts of this wiper
arm opposite, but not engaging, the corresponding row of stationary
contacts in the semi-cylindrical bank. Likewise, through the ratchet
cylinder on the intermediate portion of the shaft, the magnet shown at
the right-hand portion of this figure will, when energized by a
succession of electrical impulses, rotate the shaft a step at a time, as
many steps as there are impulses. This will thus cause the contacts of
the wiper arm to move over the successive contacts in the row opposite
to which the wiper had been carried in its vertical movement.

[Illustration: Fig. 380. Principles of Automatic Switch]

At the lower left-hand corner of this figure, there is shown a pair of
keys either one of which, when operated, will complete the circuit of
the magnet to which it is connected, this circuit including a common
battery. In a certain rough way this pair of key switches in the lower
left-hand corner of the drawing may be taken as representing the
call-transmitting apparatus at the subscriber's station, and the two
wires extending therefrom may be taken as representing the line wires
connecting that subscriber's station to the central office; but the
student must avoid interpreting them as actual representations of the
subscriber's station calling apparatus or the subscriber's line