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.