80 



Barns — Displacement In terferometer, 



in the voltmeter as in the electrometer. The values of h given 

 for this table show how nearly a true condition of symmetry 

 was attained, as its mean value from the equation 



v:= 



Mff D> 



It 1 . 



Ai^ 



(-4)' 



is but — - 0085 centimeter. 



The data, as a whole, illustrate the peculiarities of the disk 

 method very well, giving evidence both of its relatively great 



Fig. 7. 



m 40 60 80 



Fig. 7. Electrometer data compared with actual voltages. 



sensitiveness (the double displacements being even 2 X 10" 3 centi- 

 meter per volt) on the one hand, and the variation of sensitive- 

 ness as the result of a more or less unsymmetric position of 

 the disk on the other. The ratio of observed and actual volt- 

 ages is fairly constant. The question then arises as to the 

 degree to which the latter difference may be corrected. There 

 is, in many cases, a peculiar shift of the zero of displacement 

 for the uncharged apparatus which is not easily accounted for ; 

 though, from another point of view, it is truly astonishing 

 that a suspended disk should adjust itself to a given position 

 with an accuracy comparable with the wave length of light. 

 In the experiments detailed, such difficulties were eliminated 

 by taking mean results, but usually the position in question 

 was actually stable. If the shortcomings can be overcome, the 

 practical limits of the apparatus as here constructed should, for 

 short suspension (23 centimeters), be about '015 volt per vanish- 

 ing ring. 



