THE ELECTRIC CURRENT 



141 



Experiment 100. Attach a piece of copper wire to a strip of 

 zinc, and another piece to a strip of copper. Nearly fill a tumbler 

 with water and pour into it a little sulphuric acid. Put the two 

 metal strips into the water, being very careful that they do not 

 touch each other at any point (Fig. 103). Now touch the ends of 

 the wires together, as in the figure, hold the wire over a compass 

 needle, and note any movement of the needle. This device is a 

 simple cell, which may produce 

 a weak current of electricity. 



A cell consists of two 

 different solid conductors 

 placed in any liquid con- 

 ductor (except in a fused 

 metal). It is found that 

 when two such solid con- 

 ductors are placed in such 

 a liquid, they will be 

 charged, but with a differ- 

 ence in the potential of 

 each. If the solid bodies 

 are of zinc and copper, as 

 in Experiment 100, and the fluid is sulphuric acid in 

 water, the potential of the copper strip will be higher 

 than that of the zinc. 



Now we have learned that the charge upon a con- 

 ductor covers its whole surface, and spreads at once to 

 any other conducting surface which touches it ( 157). 

 Thus if the two strips are not allowed to touch each 

 other, and a wire of some conductor (copper) joins them 

 outside the liquid, the charge on the copper strip (having 

 the higher potential) will at once discharge along the 

 conducting wire to the zinc strip, which has the lower 



FIG. 103 



