112 EXPERIMENTS WITH THE DISPLACEMENT INTERFEROMETER. 



ring. The advantage of wide quadrants is thus again sustained. The sensi- 

 tiveness, however, lags behind the result for the large biplane needle III 

 (curve g) under the same circumstances. 



The intermediate biplane needle II in the quadrants III shows the results 

 of table 18. 



TABLE 18. Needle II. Quadrants III. Copper frame. 



The sensitiveness is low, owing again, no doubt, to the position of the 

 needle. Otherwise the observations are good. The needle was then raised, 

 with the following results : 



TABLE 1 8. Continued. 



The sensitiveness has been slightly increased. Moreover, it grows larger in 

 the course of the work, as if some surface viscosity in the liquid of the damper 

 were gradually overcome. The instrument in general behaved admirably, 

 barring alone the presence of drift which can not in the present laboratory be 

 quite overcome. As the fibers were but 13.5 cm. long as compared with 23.0 

 cm. above, the mean reduced sensitiveness was 25 microvolts per ring. It 

 is thus inferior both to the large biplane and to the wedge, for reasons which 

 do not appear. The sensitiveness should have been intermediate. 



Finally, the intermediate quadrants II were again mounted with the same 

 needle, the quadrants being specially smoothed inside, so as possibly to elimi- 

 nate electric restoring forces. The results, however, were not essentially 

 different from the above. 



74. Summary. The results of this long and excessively laborious paper 

 may be given in a few words. By providing the needle of the quadrant elec- 

 trometer with a pair of mirrors, in parallel, and observing displacements on 

 the interferometer, voltages as small as 10 microvolts may be detected per 

 vanishing interference ring, so that a single microvolt should be reached by 

 estimation. In the above experiments this could not be done, because the 

 needle was never confined to a fixed position of equilibrium, to an extent com- 

 patible with the use of light-waves. The causes of this drift are incidental, 



