EXPERIMENTS WITH THE DISPLACEMENT INTERFEROMETER. 105 



light might pass from mirror to mirror and return through the inside of the 

 quadrants. For since the mirrors are quite inclosed in the latter, the path of 

 the beam of light can not easily be seen, and it is troublesome to obtain the 

 several reflections to the best advantage. As the needle fits the quadrants 

 with but one-eighth inch of clear space, it is very liable to be unstable if the 

 parts of it are but slightly out of true. 



69. Observations, continued. For this reason it was thought preferable 

 to conduct the experiments by using a needle provided with mirrors attached 

 on the outside of the quadrants. Such a needle (No. i) is shown in fig. 64 

 (of the wedge type) at cd, the two 8 -shaped leaves meeting on the outside, 

 in a horizontal circular arc xy. The mirrors with the axles in cork are shown 

 at ef, and should be several centimeters above the quadrants ab. The adjust- 

 ment here is comparatively easy, as the mirrors and the path of light are all 

 quite visible. The needle, being sharp-edged, may be 



charged to a potential of several hundred volts, with- 

 out instability. The period, however, is still large. 



In the first series of experiments the needle was 

 charged with a Zamboni cell to about 150 volts, and 

 the voltage measured at the quadrants was about 

 0.04 volt. The ellipses showed continual drift, the 

 needle moving as if a force acted in one direction 

 for a time large as compared with the period of the 

 needle. The mirrors were slightly curved, so that in FlG - 6 4- 



place of ellipses the interference figures were lemniscates. In spite of the diffi- 

 culties, the two series of experiments show sensitiveness of 0.5 and 0.4 cm. 

 per volt, respectively, which is equivalent to about 7 X icH volt per vanishing 

 interference ring. 



Using the same needle, the voltage was now presumably doubled by using 

 two Zamboni piles. The sensitiveness, however, not only was not enhanced, 

 but showed a decrease, 0.02 volt being measured. In other experiments the 

 sensitiveness was successively 0.5, 0.4, 0.4 cm. per volt, respectively. 



70. Observations, continued. The sensitiveness was now increased by 

 using a new needle (II) of the form given in figs. 6$A and 656. The two 8- 

 shaped leaves or biplanes, c and d, of the needle are parallel and the circular 

 edges at x and y closed with parts of cylindrical shells of aluminum foil. It 

 is presumable from the elementary theory of the instrument that these walls 

 x, y must contribute essentially to its sensitiveness. In the present case the 

 capsule of the quadrants (II) was about 10 cm. in diameter and about 2 cm. 

 in vertical height, within, with a length of the needle, xy, of about 9 cm. and 

 a distance apart of the biplanes c and d about 0.8 cm. This gave about 0.5 

 cm. of clear space at the ends and about 0.6 cm. of clear space above and below 

 the needle, as an allowance for stability. The needle swung freely and was 

 inserted without difficulty. The mirrors were about 8 cm. apart. To secure 

 greater steadiness a water damper was installed below, though it would not 



