HOW ELECTRICITY MAY BE LOST TO USE 321 



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 Fig- 

 ure 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 



. . FIG. 216. Push button. 



the proper connection at 5. 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. 



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 pur- 

 chase 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 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, 



CL. GEN. SCI. 21 



