6o4 



THE POPULAR SCIENCE MONTHLY 



whose results accord with those of Ohm. Sir Humphry Davy about 

 1820 used a voltaic pile and a divided circuit, one branch of which 

 contained apparatus for the decomposition of water and the other the 

 wire under test. 



Fig. 1 shows the disposition of the apparatus. 



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The experiment consisted in adjust- 

 ing the length of the wire a-6 until its 

 shunting effect was such as to make the 

 potential difference across the water- 

 cell just sufficient to cause electrolysis 

 to begin. He found that wires having 

 the same ratio of length to cross-section 

 liad the same resistance. This fact, 

 while in accordance with Ohm's law, is 

 a necessary but not a sufficient condi- 

 tion for its establishment. It is, in fact, 

 the same result that Cavendish had 

 reached forty years before. 



Becqverel: Antoine Cesar Becquerel, 

 the first of an illustrious line of French 

 ]3hysicists, was the discoverer of part 

 II. of Ohm's law. As his rival. Ohm 

 must certainly have been incited by him 

 to greater efforts in his own study of conduction, and in his earliest pub- 

 lished papers Dr. Ohm accords to the work of Becquerel both recognition 

 and criticism. Becquerel, like all of the predecessors of Ohm, over- 

 looked the significance of the internal resistance of the source of cur- 

 rent, but like Davy his use of a null method eliminated the necessity of 

 taking it into account. 



In his experiment Becquerel wound two wires simultaneously on to 

 the frame of a '' multiplier " (galvanometer). The terminals of the two 

 coils thus formed being brought out separately could be connected so as 

 either to increase or to oppose each other's effect. In the latter case 

 what is known as a differential galvanometer is formed, the first one on 

 record. Becquerel connected the coils differentially and in parallel. In 

 order now that the coils shall exactly balance each other it is necessary 

 not only that the number of turns of wire be the same on the two coils, 

 but also that their resistance be the same. Since, however, the wires were 

 of different diameter this could only be accomplished by increasing the 

 length, outside of the instrument, of the coil of lower resistance. From 

 one set of experiments Becquerel found, as had Davy before him, that 

 all wires having the same ratio of length to cross-section had the same 

 resistance. But to this experiment he added another showing that the 

 conducting power varied inversely as the length of the conductor. The 



