120 MODERN ELECTRICAL INVENTIONS 



P (Fig. 89). That is, the pressure on the push button permits 

 current to flow to the bell. The flow of this current depends 

 solely upon the connection at P, which is alternately made and 

 broken, and in this way produces sound. 



The sign " Bell out of order " is usually due to the fact that 

 the battery is either temporarily or permanently exhausted. 

 If wet cells are used, the liquid may dry up in warm weather 

 and stop the current or the liquid may have eaten up all the 



zinc. These difficulties may be 

 remedied by adding water or by 

 renewing the zinc. If dry cells are 



FIG. 90. Pushbutton. used there is no remedy except in 

 the purchase of new cells. 



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 sub- 

 stances 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-conduc- 

 tors, or, more commonly, insulators. Substances which, like 

 the earth, the human body, and all other moist objects, con- 

 duct 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 elec- 

 tricity would escape into surrounding objects, instead of flow- 

 ing through the wire and producing sound and light. 



In our city streets, the overhead wires are supported on 

 glass knobs or are closely wrapped with a non-conducting 

 material, HI order to prevent the escape of electricity through 



