242 PHYSICS: C. BARUS Proc. N. A. &. 
also a report on "Data Relating to X-Ray Spectra" by William Duane, published by 
the National Research Council. 
3 Blake and Duane, Physic. Rev., Dec., 1917, p. 624. 
4 See B. Wagner, Jahrbuck der Radioahctivitat, 1919, also Physik. Zeit., Nov., 1920, 
p. 621; and C. Zecker, Ann. Physik. Leipzig, Sept., 1920, p. 28, also a note by D. L. 
Webster, presented to the American Physical Society at the same meeting at which 
the authors presented a note on this research, April, 1921. 
THE U-TUBE ABSOLUTE ELECTROMETER 
By Cari, Barus 
Brown University 
Communicated May 25, 1921 
1. Electrical Condenser. — Adjusting the wide shank of the shallow U-tube 
heretofore described (these Proceedings, 7, 1921, p. 71) with the top plates 
removed, so as to admit a metallic disc above the earthed mercury surface 
and parallel to it, the device becomes an absolute electrometer. The disc 
is perforated at the middle so that the component rays of the interfer- 
ometer may reach the mercury. This instrument is chiefly useful in 
measuring electrostatic potentials. If p is the electric-pressure below the 
disc charged at potential difference V and h is the head of mercury result- 
ing 
V = d ^8TTp — dyl87rhpg = d yj^ir\pg.n 
where d is the distance between disc and mercury, p the density of mercury 
and X the wave-length of light when n fringes correspond to V. Hence if 
d=l millimeter, V=.315 els. units; or =95 volts. 
2. Improved Apparatus. — The electrometer eventually took the form shown 
in figure 1 which gives the apparatus in connection with the electrophorus 
1-0 -8 -6 -8 40 1-1 H 
and a commutating key similar to Mascart's. To put the mercury M to 
earth, a steel screw, 5, which also carries a flat clamp for fastening one end 
of the earthed wire, has been inserted. This screw, 5, has the further im- 
portant purpose of damping the oscillations of the mercury M or M', by 
adjustably closing the channel m. The deflections can thus be made quite 
dead beat, which is an advantage. To level the electrodes C, C (using a 
small spirit level placed on them) each has connecting rod d, which carries 
a clamp at one end, allowing the rod a, a' to slide up and down, rotate 
