106 EXPERIMENTS WITH THE DISPLACEMENT INTERFEROMETER. 



otherwise have been necessary. The drift was at first marked, but finally 

 subsided to a reasonably small value. The sensitiveness (centimeters of dis- 

 placement per volt) in successive groups of observations was (AV potential 

 increment, AA/" micrometer displacement) A/V/AF=i.o, i.i, i.i, 1.6, 0.8, i.o, 

 i.i, 1.2; therefore about i . i cm. per volt, equivalent to 3 X i o- 5 volt per vanish- 

 ing interference ring. In these data some extraneous oscillation of the needle 

 is manifest, which vanishes in consecutive groups of results. 



The experiment was then repeated with two Zamboni cells charging the 

 needle. This, however, again actually reduced the sensitiveness to A7V/AV = 

 0.8, 0.7, 0.6, 0.7, 0.7 cm. per volt, in successive groups of observations, a result 

 equivalent to 4 X io~ B volt per vanishing ring. The data remained of the same 

 order, whether the needle was charged from above 

 or below, so that it is inherent in the theory of the 

 instrument. On returning to the single Zamboni 

 charging cell, sensitiveness again increased to 

 AA/yAF= 1.15, or to about 26Xio~ 6 volt per ring. 



The Zamboni cells were now removed and the 

 needle charged from above with the electric- 

 lighting circuit at 250 volts. To obviate the effect 

 of drift, which is liable to be persistently in one 

 direction, observations were taken every 1.5 min- 

 utes. The sensitiveness in two groups of experi- 

 ments of about 5 observations each was then i.i 

 and 1.4 cm. per volt, or on the average 24Xio~ 6 

 volt per vanishing ring. Many other experiments 

 were made with similar results. 



The annoyance of a drifting needle, which occurs 

 throughout the above results and which at first 

 seemed to have a definite direction from the dark 

 to the light side of the parallel mirrors, was also 



FIG. 65. 



made the subject of considerable study, sunlight being used to avoid the radia- 

 tions from the body of the electric lamp. In these cases the displacement of 

 about 0.07 cm. within a half hour was usually reversed in the course of this time, 

 so as to bring the needle nearly back to its original position. As the experiments 

 were made with the apparatus uncharged, the only reason for this drift seemed 

 therefore to be the occurrence of steady air-currents, in spite of the protection 

 of the case and the rapid subsidence of the pendulum vibrations of the damped 

 needle. The attempts made to obviate these difficulties were all futile. 



71. Observations, continued. Another biplane needle (III) in place of the 

 last (figs. 6sA and 653) was now installed. The blades of the needle were 1.2 

 cm. apart and to give it stiffness a vertical partition running symmetrically 

 from end to end was fixed within, the whole being of aluminum foil 0.002 cm. 

 thick and the frame, as before, of steel wire 0.044 cm- in diameter. The weight 

 of the needle with mirrors adjusted was about 1.2 grams, the bifilar suspension 

 22 cm. long and the threads about 0.05 cm. apart. Unfortunately the damp- 



