Barus — Displacement Interferometer. 65 



Akt. VI. — Electrometry with the Displacement Interfer- 

 ometer*; by Cael Barus. 



1. Introductory. — The possibility of compensating any dis- 

 placement of the mirror JV by a corresponding displacement 

 of the micrometer M, i. e. the fact that the ellipses are never 

 lost in a properly adjusted displacement interferometer, for 

 any amount of motion of the mirror JV, however sudden it may 

 be, suggests the use of this method for electrometry. Some 

 time ago I madef experiments of this kind, using the device of 

 the absolute electrometer for the purpose. A mirror was 

 attached to the disk and the force of restitution obtained from 

 a pendulum suspension for parallel motion. It appeared that 

 the apparatus is very insensitive for small potentials, even with 

 the use of the interferometer. 



A cylindric or lamellar form, such as is usually referred to 

 in constructing the theory of the instrument, seemed to be 

 specially available ; but the sensitiveness here also, if torsion 

 systems are excluded, is unexpectedly small, unless (as in 

 Thomson's original apparatus) excessive charges are given to 

 the needle, a procedure which introduces its own difficulties. 

 Nevertheless I constructed the apparatus in a variety of forms, 

 each of which will be described, in turn, in order to ascertain 

 in how far the sensitiveness, which may be estimated from 

 theory, may be actually realized in practice ; i. e., to actually 

 confront the instrumental difficulties of the problem, with a 

 view to the use of the method in absolute measurements of 

 potential. 



2. Cylindric electrometer. Movable cylinder without. — 

 Figs. 1 and 2, both sectional elevations, show the first form of 

 electrometer in which the movable cylinder (usually highly 

 charged) is external to the fixed cylinders h and i carrying- 

 opposite charges. The latter are supported on hard rubber 

 pillars, u and v, rising from the adjustable feet q, of which r 

 and s are the leveling screws and the set screw. The whole 

 arises from a narrow base of brass plate AA. On this base 

 the braced scaffolding of very thin brass pipe efg is also 

 mounted, grasped above by the light cross rod d. This makes 

 a light but very firm support for the two parallel horizontal 

 rods of hard rubber cc, secured at b and carrying the revoc- 

 able brass rollers or reels aa snugly. From aa the threads of 

 very thin copper wires yy -007 crn in diameter depend, their 

 lowest point being attached to the cylinder kk, making of it a 

 pendulum. To obviate torsional vibration the wires yy con- 



* Abridged from a Report to the Carnegie Institution, Washington, D. C. 

 f Physical Review, iv, p. 400, 1897. 



Am. Jour. Sci. — Fourth Series, Vol. XXXVII, No. 217.— January, 1914. 

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