CHAP, in.] ELECTRICITY AND MAGNETISM. 127 



of metal. Where the copper wire is joined to the zinc 

 there is contact of dissimilar metals, and, as we saw on 

 p. 69, the copper will become , while the zinc in con- 

 tact with it becomes + . If, now, the electrical conditions 

 of the cell are examined, we shall find that the copper 

 wire is - , while the zinc plate, the liquid, the copper plate, 

 and the wire attached to it, are all nearly at one potential, 

 and all +, as compared with the copper wire that 

 touches the zinc. There is a difference of potential 

 then between the two copper wires, one being + the 

 other . Wherever there is a difference of electric 

 potential there is a tendency for the electricity to flow 

 in a current from the place of high potential to the place 

 of low potential. Electricity cannot, however, flow back 

 through the cell against the contact -force which has 

 produced this condition of things, so it remains as it is, 

 one terminal or pole (that fixed to the copper) being 

 -}- , and the other pole (that fixed to the zinc) being . 

 If, however, the two copper wires be made to touch one 

 another, the electricity will flow through the wires from 

 the -f- towards the . It is found, moreover, that the 

 flow of electricity goes on continuously, so long as the 

 wires are joined ; for there are set up certain chemical 

 and electrical actions in the cell, (which we will here- 

 after more fully explain) which have the result of 

 renewing the difference of potential, and supplying 

 electricity to the + pole just as fast as that electricity 

 flows away through the wire to the pole. In fact, 

 there is a continuous current of electricity flowing through 

 the cell from the zinc to the copper, and from the copper 

 through the conducting wire back to the zinc. At the 

 same time it will be noticed that a few bubbles of 

 hydrogen gas appear on the surface of the copper plate, 

 and more make their appearance as long as the external 

 wires are joined. It will be found also that the zinc 

 strip is slowly dissolving in the dilute acid, and that this 

 action also goes on as long as the wires are joined. 



