Page:Popular Science Monthly Volume 83.djvu/608

604 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.

The experiment consisted in adjusting the length of the wire a-b 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 had the same resistance. This fact, while in accordance with Ohm's law, is a necessary but not a sufficient condition for its establishment. It is, in fact, the same result that Cavendish had reached forty years before.

Becquerel: Antoine Cesar Becquerel, the first of an illustrious line of French physicists, 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 published papers Dr. Ohm accords to the work of Becquerel both recognition and criticism. Becquerel, like all of the predecessors of Ohm, overlooked the significance of the internal resistance of the source of current, 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