S8§ MEASUREMeMT^ of TitE RfiStST^AlJCE 



as if tlie electrolytic cell were not behaving as a voltaic 

 cell. 



Both Ewing and MacGregor, and Prof. MacGregor overcame 

 this difficulty in this way. Call e the E. M. F. of the cell. Pass 

 a current through the cell in such a direction as to produce a 

 polarization current greater than, and opposite in direction to, 

 that produced by e. Then, as this polarization slowly dies away, 

 there will be a short time during which it will neutralize e. 

 During that time they considered that the test could be made 

 without error due to e. 



To this procedure there are objections. In order to produce the 

 necessary polarization so that it will die away slowly, the current 

 must pass for a certain time, and thus a certain amount of elec- 

 trolysis goes on — the more, the stronger the current. Then there 

 is the great difficulty in making the test at the instant at which 

 e is exactly neutralized. In my very many trials, I am not cer- 

 tain that I succeeded in making one test with which I was per- 

 fectly satisfied. In any case we do not entirely eliminate error 

 due to e. For we have neutralized e only by covering our elec- 

 trodes with gas, which must make the resistance of our cell 

 appear greater than it i^ally is. Since, however, e can always 

 be made very small, this error is, in practice, negligible. 



In his paper. Prof. MacGregor pointed out one result of which 

 he made no use, but which suggested to me an easier solution of 

 this difficulty. What he showed was, in effect, that if the Gal- 

 vanometer wire be joined up first and the spot of light be allowed 

 to take up a new position due to e, and if the adjustment of the 

 arms corresponds to the exact resistance of the cell, then, on mak- 

 ing contact in the battery wire, the current through the Galvano- 

 meter due to e will remain unaltered. It seems reasonable to 

 suppose therefore that, if the adjustment of the arms nearly cor- 

 responds to the exact resistance of the cell, and if e be very 

 small, then, under these circumstances, any change in the 

 current due to e flowing through the Galvanometer, which occurs 

 when contact is made in the battery wire, will be very small. If 

 then we should join up the Galvanometer wire first and make our 



