EXPERIMENTS WITH THE DISPLACEMENT INTERFEROMETER. Ill 



but the displacements begin with a potential of 0.009 y olt. Allowing for this, 

 the sensitiveness is 66 microvolts per vanishing interference ring. The steadi- 

 ness of the needle in both series of experiments was exceptional for this labora- 

 tory, so that the motion of single rings could at times have been counted. 

 Nevertheless the relative values of successive displacements for the same 

 potential increment were not superior to the above. 



Since di = dd/2a sin i, the electrometer itself in the most sensitive detailed 

 case (needle III, quadrant III) was only moderately sensitive, for if *' = 45, 

 = 4.5 cm., d5/AF = A/V/AF=2 or At/AF = o.32 radian per volt=i8.4 per 

 volt. Hence, the reflected ray in the ordinary mirror and scale adjustment, 

 at i meter distance of scale from mirror, would move over about 64 cm. per 

 volt. In one of the incidental cases above, it is true, about three times this 

 value was reached. In the other cases it was proportionately less sensitive. 

 Thus for A/V/AF = o.5, the deflection would be but 16 cm. per volt. No doubt 

 much could be accomplished by making the electrometer itself more sensitive ; 

 but this improvement was not the immediate purpose of the present article. 



Other comparative experiments with copper-framed needles were now made. 

 The sharp-edged needle, I, placed in the large quadrants III gave the results 

 of table 1 7 . 



TABLE 17. Needle I. Quadrants III. Copper frame. 



In this case the displacements are not proportional to the voltages, but 

 increase at an accelerated rate. Neither do they seem to begin at the origin. 

 The sensitiveness accordingly increases rapidly with increased deflection, but 

 its mean value is of the ordinary magnitude. This behavior of a thin needle, 

 in relatively wide quadrants, where stability should have been insured, was 

 unexpected; but by raising the needle the following results were found, showing 

 that the needle above was inadequately centered: 



TABLE 17. Continued. 



This result is a great improvement ; for not only is the potential proportional 

 to the displacement (see curve i, fig. 66), but the sensitiveness is much larger 

 than heretofore, for the same needle, being about 30 microvolts per vanishing 



