the point of
entry, and the contact will frequently fail. If it be necessary to have
a mercury contact, the pin must be well driven in below the surface of
the mercury or else swept through it as an oar is swept through the
water. Another form of electrical contact that acts well is a knife edge
brought into contact with a series of fine elastic strips of metal laid
parallel to one another like the fingers of a hand. The best metal for
contacts, if they are to bear hard usage, is either silver or gold or a
mixture of 40% iridium with 60% of platinum. A pressure of some 15
grammes, at least, is needful to secure a good contact.

[FIG. 28.]

As to the source of current for driving electrical clocks, if Leclanche
cells be used they should preferably be kept in the open air under cover
so as not to dry up. If direct electric current is available from
electric light mains or the accumulators used for lighting a private
house, so much the better. Of course the pressure of 50 or 100 volts
used for lighting would be far too great for clock-driving, where only
the pressure of a few volts is required. But it is easy by the insertion
of suitable resistances, as for instance one or more incandescent lamps,
to weaken down the pressure of the lighting system and make it available
for electric clocks, bells or other similar purposes.

Electricity is applied to clocks in three main ways:--(1) in actuating
timepieces which measure their own time and must therefore be provided
with pendulums or balance wheels; (2) in reproducing on one or more
dials the movements of the hands of a master clock, by the aid of
electric impulses sent at regular intervals, say of a minute or a
half-minute; and (3) in synchronizing ordinary clocks by occasional
impulses sent from some accurate regulator at a distance.

Electrically driven timepieces may be divided under two heads:--(a)
those in which the electric current drives either the pendulum or some
lever which operates upon it, which lever or pendulum in turn drives the
clock hands; and (b) those timepieces which are driven by a weight or
spring which is periodically wound up by electricity--in fact electrical
remontoires.

[Illustration: FIG. 29.--Electrical Clock (faulty design).]

The simplest clock of the first character that could be imagined would
be constructed by fastening an electromagnet with a soft iron core to
the bottom of a pendulum, and causing it to be attracted as the pendulum
s