220 



Common Science 



FIG. 123. 



Electricity flows around a completed circuit somewhat as water might 

 be made to flow around this trough. 



and back again corresponds to the tank. The elec- 

 tricity corresponds to the water. Your dynamo pushes 

 the electricity around and around the circuit, as the 

 paddle pushes the water. But let some one break the 

 circuit by putting a partition between two parts of it, 

 and the electricity immediately stops flowing. One of 

 the most effective partitions we can put into an electric 

 circuit is a gap of air. It is very difficult for any elec- 

 tricity to flow through the air ; so if we simply cut the 

 wire in two, electricity can no longer flow from one part 

 to the other, and the current is broken. 



Breaking and making the circuit. The most con- 

 venient way to put an air partition into an electric cir- 

 cuit and so to break it, or to close the circuit again so it 

 will be complete, is to use a switch. 



Experiment 67. In the laboratory, examine the three 

 different kinds of switches where the electricity flows into 

 the lamp and resistance wire and then out again. Trace 

 the path the electricity must take from the wire coming into 

 the building down to the first switch that it meets; then 



