Alexander Bain was the first to drive a clock with electricity instead of weights, by employing a pendulum having an iron bob, which was attracted to one side and the other by an electromagnet, but as its rate depends on the constancy of the current, which is not easy to maintain, the invention has not come into general use.  The “butterfly clock” of Lemoine, which we illustrate in figure 89, is an improved type, in which the bob of soft iron P swings to and fro over the poles of a double electro magnet M in circuit with a battery and contact key.  When the rate is too slow the key is closed, and a current passing through the electromagnet pulls on the pendulum, thus correcting the clock.  This is done by the ingenious device of Hipp, shown in figure 90, where M is the electromagnet, P the iron bob, from which projects a wire bearing a light vane B of mica in the shape of a butterfly.  As the bob swings the wire drags over the hump of the metal spring S, and when the bob is going too slowly the wire thrusts the spring into contact with another spring T below, thus closing the circuit, and sending a current through the magnet M, which attracts the bob and gives a fillip to the pendulum.

Local clocks controlled from a standard clock by electricity have been more successful in practice, and are employed in several towns—­for example, Glasgow.  Behind local dials are electromagnets which, by means of an armature working a frame and ratchet wheel, move the hands forward every minute or half-minute as the current is sent from the standard clock.

The electrical chronograph is an instrument for measuring minute intervals of time by means of a stylus tracing a line on a band of travelling paper or a revolving barrel of smoked glass.  The current, by exciting an electromagnet, jerks the stylus, and the interval between two jerks is found from the length of the trace between them and the speed of the paper or smoked surface.  Retarded clocks are sometimes employed as electric meters for registering the consumption of electricity.  In these the current to be measured flows through a coil beneath the bob of the pendulum, which is a magnet, and thus affects the rate.  In other meters the current passes through a species of galvanometer called an ampere meter, and controls a clockwork counter.  In a third kind of meter the chemical effect of the current is brought into play—­ that of Edison, for example, decomposing sulphate of copper, or more commonly of zinc.

The electric light is now used for signalling and advertising by night in a variety of ways.  Incandescent lamps inside a translucent balloon, and their light controlled by a current key, as in a telegraph circuit, so as to give long and short flashes, according to the Morse code, are employed in the army.  Signals at sea are also made by a set of red and white glow-lamps, which are combined according to the code in use.  The powerful arc lamp is extremely useful as a “search light,” especially on men of war and fortifications, and it has also been tried in signalling by projecting the beam on the clouds by way of a screen, and eclipsing it according to a given code.