CHAP, in.] ELECTRICITY AND MAGNETISM. 



129 



For brevity we sometimes write it E.M.F. In this 

 particular case it is obviously the result of the difference 

 of potential, and proportional to it. Just as in water- 

 pipes a difference of level produces a pressure, and the 

 pressure produces a flow so soon as the tap is turned 

 on, so difference of potential produces electromotive-force, 

 and electromotive-force sets up a ctirrent so soon as a 

 circuit is completed for the electricity to flow through. 

 Electromotive-force, therefore, may often be conveniently 

 ^pressed as a difference of potential, and vice versa j 

 ut the student must not forget the distinction. 



156. Volta's Laws. Volta showed (Art. 71) that 

 he difference of potential between two metals in contact 

 lepended merely on what metals they were, not on 

 Jieir size, nor on the amount of surface in contact. He 

 [lso showed that when a number of metals touch one 

 mother the difference of potential between the first and 

 .st of the row is the same as if they touched one 

 nother directly. A quantitative illustration from the 

 searches of Ayrton and Perry was given in Art. 72. 

 ut the case of a series of cells is different from that of 

 mere row of metals, for, as we have seen, when two 

 etals are immersed in a conducting liquid theyare 

 ereby equalised, or nearly equalised, in potential, 

 ence, if in the row of cells the zincs and coppers are 

 1 arranged in one order, so that all of them set up 

 .ectromotive- forces in the same direction, the total 

 \ectromotive-force of the series will be equal to the 

 \ectromotive-force of one cell multiplied by the mtmber 

 cells. 



1&7. Hitherto we have spoken only of zinc and 

 pper as the materials for a battery ; but batteries may 

 made of any two metals. That battery will have the 



_,. I 4er is moved by a magnet we speak rightly of magnetic force ; when 

 'ptricity 'moves matter we may speak of electric force. But E.M.F. is 

 , ,_je a different thing, not "force" at all, for it acts not on matter but on 

 e . , IJictricity, and tends to move it. 



