508 KEroRT— 18G/. 



electrometer, in anticipation of either remedying, or of learning to perfectly 

 allow for, the temperature error, and of finding by secnlar experiments on the 

 elasticity of metals, that their properties used in the instrument are satisfactory 

 as regards the permanence from year to year, and from century to century, of 

 the electric value of its reading. It is an instrument capable of being applied 

 with great ease to very accurate measurements of differences of potential, in 

 terms of its own unit. The value of the unit for each such standard instru- 

 ment ought, of course, to be determined with the greatest possible accuracy 

 in absolute measure ; and until confidence can be felt as to its secular con- 

 stancy, determinations should frequently ])e made by aid of the absolute 

 electrometer. 



§ 35. The Leyden jar of the standard electrometer consists of a large thin 

 white-glass shade coated inside and outside to within 6 centimetres of its lip, 

 and placed over the instrument as an ordinary glass shade, to protect against 

 dust, currents of air, and change of atmosphere. It may bo removed at plea- 

 sure from the cast-iron sole of the instrument, and then the interior works 

 are seen, consisting of 



(1) A continuous disk of brass supported on a glass stem, in prolongation 

 of a stout brass rod or tube sliding vertically in Vs, in which it is kept ,by 

 a spring, and resting with its lower fiat end on the upper end of a micrometer- 

 screw shaft, shown in fig. 13, where the screw, graduated circle, and stout 

 brass rod are as seen in the instrument ; the perforated brass disk (which is 

 intended to keep the round upper end of the screw-shaft in position) is 

 shown in section in fig. 14. 



(2) Eesting on three glass columns, a guard-plate with a square apertiire 

 in its centre, and carrying on its upper side stretching-springs and thin plati- 

 num-wii'e suspension of an aluminium balance-lever, shaped like those of the 

 gauge (§ 13) and the portable (§ 23) already described, but somewhat larger. 

 The tops of the three glass columns are rounded ; a round hole and a short 

 slot in line with this hole are cut in the guard-plate and receive the rounded 

 ends of two of the columns, which are somewhat longer than the third. 

 The flat smooth lower surface of the guard-plate rests simply on the top of 

 the third glass column. The diameter of the round hole and the breadth of 



the slot in the guard-plate mav be about of the diameter of curvature of 



the upper hemispherical rounded ends of the glass column, so that the 

 bearing portions of the rounded ends in the round hole and in the slot re- 

 spectively may be inclined somewhere about 45° to the plane of the plate. 

 This well-known but too often neglected geometrical arrangement gives 

 perfect steadiness to the supported plate, without putting any transverse 

 strain ti]Don the supporting glass columns, siich as was almost inevitable, and 

 caused the breakage of many glass stems, before mental inertia opposing 

 deviations fit'om the ordinary instrument-maker's plan (of screwing the guard- 

 plate to brass mountings cemented to the tops of the glass columns) was over- 

 come. It has also the advantage of allowing the gnard-plate to be lifted off 

 and replaced in a moment. 



(3) Principal electrode projecting downwards through a hole in the sole 

 of the instrument, and rigidly supported from above by a brass mounting 

 cemented to the top of a thick vertical glass column, connected by a light 

 spu'al spring with the lower attracting-plate moved np and down by the 

 micrometer- screw. The aperture round the principal electrode may be 

 ordinarily stopped by a perforated column of well-paraffined vulcanite pro- 

 jecting some distance above and below the aperture, which I find to insulate 



