297. The Electric Bell. The ringing of the electric bell is due to the attractive power of an electromagnet. By the pushing of a button (Fig. 215) connection is made with a battery, and current flows through the wire wound on the iron spools, and further to the screw _P_ which presses against the soft iron strip or armature _S_; and from _S_ the current flows back to the battery. As soon as the current flows, the coils become magnetic and attract the soft iron armature, drawing it forward and causing the clapper to strike the bell. In this position, _S_ no longer touches the screw _P_, and hence there is no complete path for the electricity, and the current ceases. But the attractive, magnetic power of the coils stops as soon as the current ceases; hence there is nothing to hold the armature down, and it flies back to its former position. In doing this, however, the armature makes contact at _P_ through the spring, and the current flows once more; as a result the coils again become magnets, the armature is again drawn forward, and the clapper again strikes the bell. But immediately afterwards the armature springs backward and makes contact at _P_ and the entire operation is repeated. So long as we press the button this process continues producing what sounds like a continuous jingle; in reality the clapper strikes the bell every time a current passes through the electromagnet. [Illustration: FIG. 215.--The electric bell.] 298. The Push Button. The push button is an essential part of every electric bell, because without it the bell either would not ring at all, or would ring incessantly until the cell was exhausted. When the push button is free, as in Figure 216, the cell terminals are not connected in an unbroken path, and hence the current does not flow. When, however, the button is pressed, the current has a complete path, provided there is the proper connection at _S_. That is, the pressure on the push button permits current to flow to the bell. The flow of this current then depends solely upon the connection at _S_, which is alternately made and broken, and in this way produces sound. [Illustration: FIG. 216.--Push button.] The sign "Bell out of order" is usually due to the fact that the battery is either temporarily or permanently exhausted. In warm weather the liquid in the cell may dry up and cause stoppage of the current. If fresh liquid is poured into the vessel so that the chemical action of the acid on the zinc is renewed, the current again flows. Another explanation of an out-of-order bell is that the liquid may have eaten up all the zinc; if this is the case, the insertion of a fresh strip of zinc will remove the difficulty and the current will flow. If dry cells are used, there is no remedy except in the purchase of new cells.