HEAT AND LIGHT FROM ELECTRIC CURRENTS 197 



right-hand binding posts completes the metal path. The cur- 

 rent is said to pass from the carbon of the right-hand one 

 of the three cells, flowing through the circuit to the zinc 

 of the first cell, and completes the path by passing through 

 the cells. 



If a foot of copper wire (No. 30, which is one hundredth 

 of an inch in diameter) is inserted in the binding posts and 

 the switch closed, the wire becomes red-hot and melts. We 

 must learn how to control the electric current in order to 

 prevent the wires from being destroyed. If four feet of the 

 wire is placed in the circuit, the heating effect is greatly 

 reduced. The results of common experiments with other 

 wires are given in the table below. 



Since electric wires may become hot, all electric wires in 

 buildings, automobiles, motor boats, etc. should be placed in 

 metal tubes or conduits or otherwise arranged to protect 

 against danger. 



212. The use of fuses for safety. In the electric circuit 

 there is a piece of soft metal in the fuse plug (fig. 97, D) 

 which makes an important use of the heating effect just dis- 

 cussed. The current in the circuit must pass through this 

 ribbon, which is a mixture of lead and tin or of other metals. 

 If the current becomes excessive the metal melts and thus 

 breaks the connection, but the melting occurs at a much 

 lower temperature than that of the wires in the previous 

 experiment. This provides a means of inserting a sort of 



