386 MEASUREMENT OF THE REkSISTANCE 



ment of the arms indicating the exact resistance of the cell. 

 With considerable practice we may train the eye to recognize 

 this particular hesitation, so that, within a short range of this 

 resistance of the cell, we may be able to make almost perfect 

 measurements. This was the only way in which I was able to 

 use Prof. MacGregor's method and the result was unsatisfactory. 

 For when the resistance of the cell was changed to any 

 extent, the strength of the cui-rent changed and I found that 

 my familiar hesitation was no longer that which corresponded to 

 the adjustment of the arms indicating the resistance of the cell. 

 In my experiments I noticed, however, what is clearly evident, 

 that, the stronger the currents, the greater was my double 

 deflection, that if, for certain resistances of the arms, with a 

 certain current I could get a double deflection, then with a stronger 

 current I would get a larger deflection, and, what to me was more 

 important, if, in the former case, the double deflection had just 

 vanished, in the latter case it was distinctly visible. I immedi- 

 ately tried still stronger currents and found that in every case 

 the stronger the current the nearer would the adjustment of the 

 arms, when the double deflection had just vanished, indicate the 

 true resistance of the cell. It appeared, therefore, that I had 

 merely to increase the strength of the current to make my error 

 as small as I pleased. 



In attempting this, however, diflUculties arose : 

 (1.) As the battery current was increased, so was the polariza- 

 tion current. To lessen this my electrodes were made as large as 

 possible. As I have stated above, they were about 7 cm. square. 

 I might have platinized them, after the manner of Kohlrausch ; 

 but as the method under consideration, owing to the simplicity 

 of apparatus required, is especially useful for measuring resist- 

 ances of electrolytes in ordinary laboratory work, I was more 

 interested in knowing what degree of accuracy was attainable 

 by it with apparatus whicli is always at hand. 



(2.) Since the motions of the mirror are indicated by those of 

 the spot of light reflected from it to a screen, in proportion as the 

 double deflection gets smaller and smaller, the difficulty in ob- 

 serving it gets greater and greater. Especially is this so if the 



