110 EXPERIMENTS WITH THE DISPLACEMENT INTERFEROMETER. 



it is not impossible that voltaic potential differences in the quadrants (or 

 connections) may be in question. These were of brass and nominally identical ; 

 but a difference of o.oi volt is not out of the question. Allowing for the initial 

 potential difference, the sensitiveness is about &N/AV = 2.o cm. per volt or 

 50 microvolts per vanishing interference ring. The needle was far more steady 

 than in the above cases, so that measurements could be made with reasonable 

 assurance. A curious result is thus attained: on widening the quadrants, so 

 that the distance between the quadrants and the needle is increased within 

 limits, greater sensitiveness is secured. The reason has been suggested, that 

 inasmuch as the electric forces which place the needle are now small, the latter 

 is subject to the force of the bifilar suspension only. 



TABLE 15. Large needle III in large quadrants III. Copper frame. 



Further work was done with the needle at 250 volts; but an adequately 

 stable condition of the needle could not be obtained, as it gradually crept 

 beyond the range of the interferometer. 



Finally, experiments were made with a needle of the ordinary form (I, 

 69), inclosed in the intermediate quadrants (II). The relatively sharp edges 

 of the needle should reduce the electric torque. 



TABLE 16. Needle I. Quadrants II. Copper frame. 



The sensitiveness here is not inferior to the usual cases above, being on the 

 average 46 microvolts per ring, and this in conformity with the relatively low 

 potential of the needle. The following results were obtained at higher poten- 

 tials with the same needle : 



TABLE 16. Continued. 



In spite of the much larger potential of the needle in the last series, the 

 average sensitiveness is again less, showing the same behavior as the above 

 cases. The relation of potential and displacement is linear (fig. 66, curve h), 



