The battery _B_, in Fig. 238, is a single dry cell but more commonly it is composed of two dry cells joined in series. It is connected, as shown in the figure, to the binding posts _P₁_ and _P₂_ of the vibrating mechanism, the push button _PB_ serving to make contact when the circuit is to be closed. When the button is pressed the circuit is complete from the + pole of the battery cell through the binding post _P₁_, across the contact _F_, through the spring _A_, through the magnet coils _M_, across the binding post _P₂_ and push button to the-pole of the cell. The vibration of the tapper is caused by the magnetized cores of the coils _M_. When the electric current flows through the coils of wire, the iron cores become temporary magnets. This magnetism attracts the iron armature attached to the spring _A_, and it is suddenly pulled forward with energy sufficient to cause the tapper to strike the gong. As the armature moves forward, the spring contact at _F_ is broken and the current stops flowing through the magnet coils. When the current ceases to flow in the magnet coils, the cores are demagnetized and the armature is drawn back by the spring _A_ to the original position. As soon as the contact is restored at _F_ a new impulse is received only to be broken as before. In this manner the bell continues ringing so long as the push button makes contact. The screw at _F_ is adjusted to suit the contact with the spring attached to the armature. The motion of the armature may be regulated to a considerable degree by this adjustment. When properly set the screw is locked in place by a nut and should require no further attention. [Illustration: FIG. 239.--Example of an electric doorbell installation.] Electric bells vary in price according to design and workmanship. A bell outfit may be purchased complete for $1 but it is advisable to install a bell of better construction, as few pieces of household mechanism repay their cost in service so often as a well-made bell. The bell should be rigid, well-constructed, and the contact piece _F_ should be adjustable. This part _F_, being the most important of the moving parts of the bell, is shown separately in Fig. 240. Only the ends of the magnet coils with their cores are shown in the figure. The contact is made at _A_, by the pressure of the spring against the end of the adjustable screw _D_. When the screw is properly adjusted it is locked securely in place by the nut _G_. The screw _D_ is held with a screw-driver and the nut _G_ forced into position to prevent any movement. If the screw is moved, so that contact is lost at _A_, the bell will not ring. In the better class of bells the point of the screw and its contact at _A_ are made of platinum to insure long life. With each movement of the armature a spark forms at the contact which wears away the point, so that to insure good service these points must be made of refractory material.