CHAPTER V. 



DISPLACEMENT INTERFEROMETRY APPLIED TO THE QUADRANT 



ELECTROMETER. 



67. Apparatus. In an earlier report experiments were given showing the 

 adaptation of the quadrant electrometer for the measurement of very small 

 potential differences, when the needle is provided with two symmetrical, light, 

 plane mirrors, in parallel. The excursions of the needle may be read off, for 

 small angular deviation, on the displacement interferometer. If 5 = A A/" is the 

 displacement of the mirror of the micrometer of this instrument, and i the 

 angle of incidence of the ray impinging on either of the small parallel mirrors 

 on the needle, 



5 = 20, cos i d8/di= za sin * 



where a is the normal distance apart of the parallel mirrors. If degrees of arc 

 are used the ratio is 0.035 a sm * an d i is usually about 45. It should be pos- 

 sible with such an arrangement to obtain a sensitiveness of a few millionths 

 volts per vanishing interference ring, and the following paper is a further 

 attempt to reach this result, practically. 



The main, if not insuperable, difficulty encountered in such an apparatus 

 is the continual and often irregular drift of the needle, when the condition of 

 rest is so sharply determined. A special environment, without city tremors 

 and at constant temperature, seems to be the only means of obviating these 

 annoyances. 



The apparatus used is shown in fig. 63 in vertical section. AA is the per- 

 forated base of a massive brass plate, i cm. thick, securely fastened by a large 

 clamp to one arm of the interferometer, capable of some rotation around the 

 vertical and horizontal axes for leveling the whole apparatus, etc. To this the 

 quadrants a, b are firmly attached, by aid of screws i, j, but in such a way as 

 to be quite insulated from the brass plate, in view of the perforated columns 

 g, h and nuts u, v of hard rubber and of the form shown. The clamp-screws 

 k, I are in metallic contact with i, j, and carry charges to the quadrants. There 

 are about 2 inches of free space below the plate A A, available for the connec- 

 tions and, if necessary, for a liquid damper, w. 



The needle consists of two 8 -shaped leaves, c and d (biplanes), symmetri- 

 cally fastened to the stem st, on which the needle is bifilarly suspended from 

 silk fibers. The two small parallel mirrors, e and /, are adjustably attached 

 to a fine metallic wire at right angles to st and in contact with d . Each mirror 

 has a light vertical and horizontal axis in a bit of cork (not shown). The 

 mirrors are first made parallel by using sunlight and then fixed with melted 

 wax, after which the aluminum foils c and d are centered in place, the eyelets 

 at 5 and t having not as yet been bent. Light reaches the mirrors e, f through 

 two corresponding holes cut in the vertical walls of the quadrants. The 



103 



