Chap, in.] OHM'S LA iv. 27 



power of the liquid in the element being, of course, 

 always the same. Taking now the external resistance, 

 it depends on the conductivity of the conductor, 

 which is a constant quantity for each conductor. 

 Apart, however, from that, the external resistance is 

 directly proportional to the length of the conductor^ 

 and inversely proportional to the cross-section ; i.e. 

 the longer the conductor the greater the resistance, and 

 the thicker the conductor the less the resistance. 



Ohm's law. There is a relation between the 

 intensity of the current and the amount of resistance. 

 Experiment readily shows that a current due to a 

 definite difference of potentials between the extremi- 

 ties of the conductor is feebler after passing through a 

 platinum wire than after passing through the same 

 length of copper wire, because the conducting power 

 of platinum is less than that of copper. Again, a cur- 

 rent sent through a long copper wire is feebler than 

 when sent through a short one, and feebler when sent 



O ' 



through a thin than when sent through a thick one. 

 That is to say, the intensity of the current is inversely 

 proportional to the resistance, and it has been already 

 stated to be directly proportional to the electromotive, 

 force. This is the law of Ohm (so called because the 

 character of these relations was first expounded by 

 Dr. G. S. Ohm in 1827), and is put thus : 



Electromotive force 

 Current strength = - 



'& 



Resistance 



Let C stand for current strength, E for electro- 

 motive force, and E, for resistance, and the formula 

 becomes 



E 



But it has been already noted that there are two 



