EXPERIMENTS WITH THE DISPLACEMENT INTERFEROMETER. 107 



ing, even with the presence of a water well, proved insufficient and the period 

 too long in the case of the suspension used. 



An example of the six groups of results obtained successively, in the measure- 

 ment of one-fortieth volt, with the needle charged to about 150 volts may be 

 omitted, the mean being about AAf = 0.028 cm. or A./V/AV= i.ii cm. of 

 micrometer displacement per volt, equivalent to ayXio" 6 volt per ring. In 

 spite of the vertically more extended needle, there has therefore been no 

 advantage in sensitiveness over the preceding case. 



The following experiments were made at different voltages, showing better 

 agreement for the larger voltages, in which the drift is less significant, rela- 



tively. Further experiments with this needle led to no new results. In partic- 

 ular the endeavor to replace the silk suspension used by a bifilar the threads 

 of which were single fibers of silk, proved a failure owing to the instability of 

 the needle. 



72. Further observations. The same needle was now placed within large 

 quadrants (III), n cm. in diameter and 2.3 cm. high within, to obviate the 

 difficulty from instability in a needle carrying 250 volts. While this was 

 accomplished, the drift now became excessively large. The mean results were 

 AF = o.oi25 volt; AA^ = o.oy6 cm.; AA/'/AV r = 6.i cm.; or about 5Xio~ 6 volt 

 per vanishing ring. Unfortunately this large sensitiveness, the largest obtained, 

 could not be controlled. 



It appears from these results that in the above cases the actual restoring 

 torque could not have been the torsion of the bifilar, but rather a directed 

 residual electrical attraction between the needle and the quadrants, the torque 

 of the fiber being operative merely in placing the needle in the fiducial position, 

 symmetrically with respect to the division line between the quadrants. In 

 other words, the displacement of the needle is not to be estimated in terms of 

 the rate at which the bifilar torque changes per degree, but in terms of a very 

 much larger coefficient of the electrical forces in question, so that apart from 

 giving position to the needle as specified, the bifilar acts not very differently 

 from a unifilar suspension. The instrument is thus much less sensitive than 

 would be inferred from the dimensions of the bifilar. Hence it appeared de- 

 sirable to return to the needle and quadrants in 70, with the object of ascer- 

 taining whether the sensitiveness might not be actually increased by decreasing 

 the potential of the needle until a deflection fully corresponding to the torque 

 of the bifilar should show itself. The present point of view also indicates 

 why nothing was gained by the use of a larger needle in 7 1 , seeing that in 

 such a case the electrical restoring forces increase at the same rate as the 



