ELECTRIC CLOCKS
Amongst the many little worries of domestic life is the keeping
in order of the various clocks. It ought to be a very simple
matter to remember to wind up a clock, but curiously enough almost
everybody forgets to do so now and then. We gaze meditatively at the
solemn-looking machine ticking away on the mantelpiece, wondering
whether we wound it up last week or not; and we wish the wretched thing
would go without winding, instead of causing us all this mental effort.
There is usually a way of getting rid of little troubles of this kind,
and in this case the remedy is to be found in an electrically-driven
clock. The peculiar feature about clocks driven by electricity is that
they reverse the order of things in key-wound clocks, the pendulum
being made to drive the clockwork instead of the clockwork driving
the pendulum. No driving spring is required, and the motive power is
supplied by a small electro-magnet.
The actual mechanism varies considerably in different makes of clock.
In one of the simplest arrangements there is a pendulum with an
armature of soft iron fixed to the extremity of its bob. Below the
pendulum is an electro-magnet, and this is supplied with current from a
small battery of dry cells. A short piece of metal, called a “pallet,”
is attached to the rod of the pendulum by means of a pivot; and as the
pendulum swings it trails this pallet backwards and forwards along
a horizontal spring. In this spring are cut two small notches, one
on each side of the centre of the swing. As long as the pendulum is
swinging sufficiently vigorously, the pallet slides over these notches;
but when the swing has diminished to a certain point the pallet catches
in one or other of the notches. This has the effect of pressing down
the spring so that it touches a contact piece just below, and the
battery circuit is then completed. The electro-magnet now comes into
action and attracts the armature, thus giving the pendulum a pull which
sets it swinging vigorously again. The spring is then freed from the
pressure of the pallet, and it rises to its original position, so that
the circuit is broken. This puts out of action the electro-magnet, and
the latter does no further work until the pendulum requires another
pull. The movement of the pendulum drives the wheelwork, which is
similar to that of an ordinary clock, and the wheelwork moves the hands
in the usual way. A clock of this kind will run without attention
for several months, and then the battery requires to be renewed. As
time-keepers, electrically-driven clocks are quite as good as, and
often very much better than key-wound clocks.
Everybody must have noticed that the numerous public clocks in a large
town do not often agree exactly with one another, the differences
sometimes being quite large; while even in one building, such as a
large hotel, the different clocks vary more or less. This state of
things is very unsatisfactory, for it is difficult to know which of the
clocks is exactly right. Although large clocks are made with the utmost
care by skilled workmen, they cannot possibly be made to maintain
anything like the accuracy of a high-class chronometer, such as is
used by navigators; and the only way to keep a number of such clocks
in perfect agreement is to control their movements from one central or
master clock. This can be done quite satisfactorily by electricity.
The master-clock and the various sub-clocks are connected electrically,
so that a current can be sent from the master-clock to all the others.
Each sub-clock is fitted with an electro-magnet placed behind the
figure XII at the top of the dial. At the instant when the master-clock
reaches the hour, the circuit is closed automatically, and the current
energizes these magnets. The minute hands of all the sub-clocks are
gripped by the action of the magnets, and pulled exactly to the hour;
the pulling being backward or forward according to whether the clocks
are fast or slow. In this way all the clocks in the system are in exact
agreement at each hour. The same result may be attained by adjusting
all the sub-clocks so that they gain a little, say a few seconds in the
hour. In this case the circuit is closed about half a minute before
the hour. As each sub-clock reaches the hour, its electro-magnet comes
into action, and holds the hands so that they cannot proceed. When the
master-clock arrives at the hour the circuit is broken, the magnets
release their captives, and all the clocks move forward together.
It is possible to control sub-clocks so that their pendulums actually
beat exactly with the pendulum of the master-clock; but only a small
number of clocks can be controlled in this way, and they must be
of the best quality. The method is similar to that used for hourly
corrections, the main difference being that the circuit is closed by
the pendulum of the master-clock at each end of its swing, so that the
pendulums of the sub-clocks are accelerated or held back as may be
required.
In the correcting systems already described the sub-clocks are
complete in themselves, so that they work quite independently, except
at the instant of correction. For hotels, schools, and other large
buildings requiring clocks at a number of different points, a simpler
arrangement is adopted. Only one complete clock is used, this being
the master-clock, which may be wound either electrically or by key.
The sub-clocks are dummies, having only a dial with its hands, and an
electro-magnetic arrangement behind the dial for moving the hands. The
sub-clocks are electrically connected with the master-clock, and the
mechanism of this clock is arranged to close the circuit automatically
every half-minute. Each time this occurs the magnet of each sub-clock
moves forward the hands half a minute, and in this way the dummy clocks
are made to travel on together by half-minute steps, exactly in unison
with the master-clock.