ELECTRICAL INVENTIONS 225 



this copper sulphate goes into solution, but since its specific 

 gravity is high the solution remains at the bottom of the jar, 

 the salt solution above it. The zinc replaces the copper in the 

 copper sulphate solution, and the zinc sulphate ionizes. The 

 zinc thus changes from the neutral molecular condition to the 

 ionic condition with an excess of two positive charges to each 

 ion, by discharging two electrons from each atom on to the zinc 

 plate, which as this process continues becomes negatively charged. 

 The copper moves to the copper plate, and is deposited as metallic 

 copper. As it makes this change from the ionic to the molecular 

 state, it must take on electrons, drawing them from the zinc 

 plate through the connecting wire. Since the copper plate is 

 constantly giving up electrons, it has an excess of positive charges 

 and is positive. The flow of electrons is, therefore, from the 

 zinc to the copper plate. In this battery the zinc gradually 

 disappears, the copper sulphate is also used up, and crystals of 

 zinc sulphate are deposited. 



There is an instructive analogy between the flow of water 

 through pipes connected with a reservoir and the flow of electri- 

 city through the wires connected with a battery. In the former 

 the amount of water discharged depends, first, upon the head 

 of water in the tank or upon the pressure at the opening (the 

 greater the pressure, the more rapid the flow), and, second, upon 

 the character of the pipe; a long pipe diminishes the flow by the 

 friction of the water on its sides more than does a short pipe 

 of the same diameter; a pipe with rough interior reduces the 

 flow more than one with a smooth lining, and a small pipe carries 

 less water than a large one of the same material. (See p. 113.) 

 Similarly, the flow of electricity from a battery depends, first, on 

 the electric pressure developed by the battery (the greater the 

 pressure, the greater the flow) and, second, upon certain proper- 

 ties of the wire; a long wire offers more resistance than a short 

 one of the same substance and diameter; a fine wire offers more 

 resistance than a coarse one ; copper, which is a good conductor, 

 offers less resistance than iron, and both are better conductors 



