The invention of Electro-Magnetic Clocks closely followed the
introduction of the electric telegraph; and Professor Wheatstone,
to whom the world is principally indebted, in conjunction with Mr.
Cooke, for the perfection and application of the needle telegraphic
instrument, claims also to be the original inventor of Electro-Magnetic
Clocks. His claim is, however, disputed by Mr. Bain, who asserts that
he was the first who conceived the idea of applying the power of
electro-magnets to the regulation and movements of clocks, and it must
be admitted that he brought the invention into a working state.
In the first stage of the invention, the object attempted to be
attained was to regulate several clocks, once an hour--or oftener, if
required--so that they might all indicate precisely the same time.
For this purpose Mr. Bain took for a standard time-keeper a clock of
the best possible construction, placed in circumstances favourable
to maintaining accuracy. The minute-hand of his clock, the instant
that it pointed to the hour, made connection with a voltaic battery
that brought into action a series of electro-magnets attached to the
clocks to be regulated; one of them being fixed on the top of each
clock. Its momentary action was made to collapse a pair of clippers,
which in closing seized the minute-hand of the clock to which it was
attached, and brought it to the hour point. Thus all the clocks in
the series could be regulated every hour, for the collapse of the
clippers pushed the hand forward if it were too late, or thrust it
back if it had gained. Mr. Bain contemplated the application of this
contrivance to all the public clocks of a town, by having wires laid
down in the streets to connect them in one voltaic circuit. Such a
plan would, however, have involved greater expense and trouble in its
accomplishment than the object seemed to merit; but the regulation
of any number of clocks in a large establishment might have been
practicable by that means. We are not aware, however, that this mode
of regulating clocks by electricity was ever adopted, and it has since
been superseded by an arrangement made by Mr. Shepherd, junior, to be
presently noticed.
Improving on this first application of electro-magnetism to the
regulation of clocks, Mr. Bain afterwards employed the power to keep
the clocks in action, so that each clock might be propelled by magnets
alone, without any weight, and without the ordinary train of wheels.
Every one acquainted with the mechanism of a clock is aware that the
weight communicates motion to a train of wheels, and that the movement
is regulated by the vibration of a pendulum, which is acted on by
the last wheel of the train. That wheel, called the escapement, is
so formed, that each tooth catches in succession into a detent fixed
on the pendulum near the point of suspension, which allows one tooth
to pass at each double vibration. The pendulum, therefore, governs
the movement of the train of wheels by checking the escapement, and
allowing the teeth to pass one by one; and as pendulums of given
lengths vibrate in given times, if their actions be not interfered
with, the clocks will keep regular time. But the pressure of the
escape-wheel against the detent, and the consequent friction, prevent
the pendulum from acting freely. In the best made clocks there are
special contrivances to detach the pendulum as much as possible from
the wheels, and likewise to compensate for variations in the length of
the pendulum by change of temperature.
In the clocks actuated by electro-magnetism, the movement of the
pendulum is not maintained by repeated impulses of the escape-wheel, as
in ordinary clocks, but by magnetic attraction; an electro-magnet being
so arranged as to attract the bob of the pendulum in both directions
alternately. In Mr. Bain's arrangement, the bob of the pendulum
is formed of a hollow coil of covered copper wire, which, on the
transmission of an electric current, becomes magnetic, and it is then
attracted by several permanent magnets fixed in a hollow horizontal
bar, over which the coil of wire moves. The accompanying diagram will
serve to explain more clearly the parts of the clock on which the
movement of the pendulum depends.
The pendulum rod, _B_, is made of wood, and the bob, _A_, consists of a
hollow coil of thick copper wire covered with cotton, through which the
hollow bar, _C C_, passes. Inside that bar there are several permanent
magnets, packed on each side of the ends of the coil of wire, the poles
of those on one side being the opposite of those on the other. In the
diagram only one magnet on each side is represented, _n_ and _s_, to
prevent confusion. The ends of the coil of wire are attached to the
pendulum rod, and they are conducted up it so as to form connection
with the wires of the voltaic battery, which are connected with gold
studs inserted into a horizontal stage fixed to the clock-case. A small
movable bridge, formed of wire, and having the ends tipped with gold or
platinum, rests upon the stage, and is shifted from side to side by the
pendulum. In these movements the gold points touch and slide over the
gold studs in the stage, and thereby make and break contact with the
voltaic battery, and alternately send and interrupt an electric current
through the coil of wire.
[Illustration]
Suppose, for instance, that the pendulum is about to rise to the right
towards _s_, at which time the voltaic circuit is completed. The coil
is, therefore, magnetic, and is attracted by the permanent magnet in
_C_. As the pendulum approaches the end of its swing, it pushes the
movable bridge away from the gold studs on which it rests, and thus
breaks connection with the voltaic battery, and the pendulum descends
unrestrained by the attractive force of the magnets. As the pendulum
descends towards its lowest point, it shifts the bridge on to the metal
studs on the other side, which are so disposed as to send a current
through the coil in a direction opposite to the former, so that the
poles of the voltaic battery are reversed, and the attractive force
is exerted in drawing the pendulum towards the left hand. In this
manner the power imparted to the coil, as the pendulum vibrates to and
fro, produces a continuous repetition of the attraction on each side
alternately, and maintains a constant action.
The only wheels required in a clock of this kind are those which turn
the hands; and the motion is communicated from the pendulum to the
seconds wheel by means of a small attached lever, working on a ratchet
wheel. The minute and the hour hands derive their movements from the
seconds wheel in the usual manner.
The voltaic battery employed to work Mr. Bain's clocks consists of a
pair of large copper and zinc plates buried in the moist earth, which
excite a sufficient amount of electricity to maintain the motion of the
pendulum. A battery of this kind will remain in action a long time, and
will serve to keep a clock going for several months. It is, indeed, a
near approach to the attainment of perpetual motion, since nothing but
the wearing away of the materials, or the accumulation of dust on the
connecting points, seems to prevent the realization of that mechanical
chimera.
There is a disadvantage attending the arrangement of Mr. Bain's
clocks, arising from the attachment of the pendulum to the wheels;
and as the moving force is derived directly from voltaic electricity,
any variation in the power of the battery causes variation in the
lengths of the vibrations, and produces irregularity. For the purpose
of remedying these defects, Mr. Shepherd, junior, has adopted an
arrangement which detaches the pendulum from the clock movement, and
makes its vibrations altogether independent of the varying force of
voltaic batteries.
In Mr. Shepherd's arrangement, the impulse of the pendulum is given
by successive blows from a spring, which is drawn back and then
liberated at each vibration. The hands of the clock are also moved
by electro-magnets, by which means the impelling forces and the
resistances encountered by the pendulum are always constant. By making
the pendulum thus independent of the works, and employing it merely to
make and break contract at regular intervals, any number of clocks in
the same establishment may be set in motion, and kept exactly together,
by a single pendulum.
The large clock over the principal entrance to the Great Exhibition was
on this construction. It would have been impossible, with any approach
to regularity, to have moved hands of that size, exposed as they
were to the wind, unless the pendulum had been independent of such
resistances.
Electro-Magnetic Clocks have not yet come into general use, partly
owing to imperfections in the battery connections, which occasionally
put a stop to their movements, but principally on account of the high
prices charged by the patentees. As no trains of wheels are requisite
in an Electro-Magnetic Clock, it might be manufactured very cheaply;
and when the price is reduced to its proper standard, and the trifling
practical defects are remedied, these clocks may possibly supersede
others.