gh any
number of step-by-step dial movements which may be included in the same
series circuit. The armature is then drawn towards the magnet with rapid
acceleration, carrying the lever D with it. The armature is suddenly
arrested by the poles of the magnet, but the momentum of the lever D
carries it farther, and the click E engages another tooth of the ratchet
F. A quick break of the circuit is thus secured, and the contact at L is
a good one, first because the whole of the energy required to keep the
clock going, or in other words the energy required to raise the lever D
is mechanically transmitted through its surfaces at each operation, and
secondly, owing to the arrangement of the fulcrums at G and K which
secure a rubbing contact. The duration of the contact is just that
necessary to accomplish the work which has to be done, and it is
remarkable that when used to operate large circuits of electrically
propelled dials the duration accommodates itself to their exact
requirements and the varying conditions of battery and self-induction.
The ratchet wheel F is usually mounted loosely upon its arbor and is
connected to the wheel C by means of a spiral spring, which in
conjunction with the back-stop click P maintains the turning force on
the wheelwork at the instant when the lever D is being raised.

[Illustration: FIG. 33.--Hope Jones's Dial-driving Device.]

Electrically driven dials usually consist of a ratchet wheel driven by
an electrically moved pall. Care has to be taken that the pushes of the
pall do not cause the ratchet wheel to be impelled too far. The anchor
escapement of a common grandfather's clock can be made to drive the
works by means of an electromagnet, the pendulum being removed. With a
common anchor escapement the scape-wheel can be driven round by wagging
the anchor to and fro. All then that is necessary is to fix a piece of
iron on the anchor so that its weight pulls the anchor over one way,
while an electromagnet pulls the iron the other. Impulses sent through
the electromagnet will then drive the clock. If the clock is wound up in
the ordinary way the motion will be so much helped that the electric
current has very little to do, and thus may be very feeble. Fig. 33
shows the dial-driving device of Hope Jones's clock. Each time that a
current is sent by the master-clock, the electromagnet B attracts the
pivoted armature C, and when the current ceases the lever D with the
projecting arm E is driven