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.

299.  How Electricity may be lost to Use.  In the electric bell, we saw that an air gap at the push button stopped the flow of electricity.  If we cut the wire connecting the poles of a battery, the current ceases because an air gap intervenes and electricity does not readily pass through air.  Many substances besides air stop the flow of electricity.  If a strip of glass, rubber, mica, or paraffin is introduced anywhere in a circuit, the current ceases.  If a metal is inserted in the gap, the current again flows.  Substances which, like an air gap, interfere with the flow of electricity are called non-conductors, or, more commonly, insulators.  Substances which, like the earth, the human body, and all other moist objects, conduct electricity are conductors.  If the telephone and electric light wires in our houses were not insulated by a covering of thread, or cloth, or other non conducting material, the electricity would escape into surrounding objects instead of flowing through the wire and producing sound and light.