Barus — Displacement Interferometer. 83 



shifting the disk as a whole in the direction of its axis and 

 with more elaborate means for sighting with a view to hori- 

 zontal and vertical parallelism of the disk with the plates of 

 the condenser than was the case with the improvised apparatus 

 here treated. It has been stated that it is an inversion of the 

 present displacement which is superimposed on the displace- 

 ment in case of a permanently charged needle, and which thus 

 demands an apparatus tested for absence of non-symmetry, if 

 the voltages are to be proportional to the displacements. 



16. Displacement Interferometry Applied to the Quadrant 

 Electrometer. — The method of measuring small angles given 

 elsewhere may be made use of in measuring very small volt- 

 ages or small increments of potential, by attaching a pair of 

 light mirrors, symmetrically and parallel, to the needle of a 

 quadrant electrometer. In the present experiments this was 

 an improvised instrument, constructed by myself, the quad- 

 rants being of sheet copper fitted and soldered together and 

 supported on cylinders of hard rubber. The bottom of the 

 stem of the needle was submerged in sulphuric acid as in 

 Kelvin's instrument and the suspension was bifilar. The 

 insulation was throughout excellent. The needle was kept 

 charged to about 150 volts with a Zamboni pile, any variation 

 of charge being indicated by Elster and Geitel's electroscope. 



In fig. 8, qq' shows a pair of quadrants in vertical section, 

 E the needle on the stem ss', the lower end of which is 

 platinum, bent as shown, thus making a clip to hold the light 

 mica vane v (if necessary) submerged in sulphuric acid of the 

 vessel c charged by the Zamboni pile. Rli are the hard rub- 

 ber supports of the quadrants. At a suitable distance below 

 them the light parallel mirrors m and n (less than 1 centimeter 

 in diameter each) are supported by the light cross piece of 

 hard rubber rr attached to the stem ss' of the needle. The 

 axial line of the needle i^is parallel to the line rr between the 

 mirrors and the latter are placed at a horizontal angle of about 

 45° to rr. 



To adjust m and n to adequate parallelism, each is supported 

 by an attached fine needle, fitting snugly in a vertical groove 

 in the ends of rr. The needle, as a whole, is to be clamped at 

 rr in a suitable support and sunlight is to be used. When the 

 horizontal beam reflected from m to n and thence to a distant 

 white screen falls within the direct shadow of n, the needles 

 are fixed in place by resinous cement. The mirrors should be 

 equally high. 



The quadrant electrometer with the needle in position was 

 now placed, with the aid of three long foot screws, on a circular 

 platform just below the iron arm holding the fixed inter- 

 ferometer mirror M, and the reflection took place as shown in 

 fig. 9. In effect the arc light from a collimator reflected from 



