ON STANDARDS OF ELECTRICAL RESISTANCE. 489 



V. Report on Electrometers and Electrostatic Measurements. 

 By Sir Wm. Thomson, F.B.S. 



§ 1. An electrometer is an instrument for measuring differences of electric 

 potential between two conductors through effects of electrostatic force, and is 

 distinguished from the galvanometer, which, of whatever species, measures 

 differences of electric potentials through electromagnetic effects of electric 

 currents produced by them. ^V^len an electrometer merely indicates the 

 existence of electric potential, without measuring its amount,* it is commonly 

 called an electroscope ; but the name electrometer is properly applied when 

 greater or less degrees of difference are indicated on any scale of reckoning, 

 if approximately constant, even during a single series of experiments. The 

 first step towards accurate electrometry in every case is to deduce from 

 the scale-readings numbers which shall be in simple proportion to the dif- 

 ference of potentials to be determined. The nest and last step is to assign 

 the corresponding values in absolute electrostatic measure. Thus, when for 

 any electrometer the first step has been taken, it remains only to determine 

 the single constant coefficient by which the numbers deduced from its indica- 

 tions as simply jiroportional to differences of potential must be multij)lied to 

 give differences of potential in absolute electrostatic measure. This coefficient 

 will bo called, for brevity, the absolute coefficient of the instrument in question. 



§ 2. Thus, for example, the gold-leaf electrometer indicates differences of 

 potential between the gold leaves and the solid walls enclosing the air-space 

 in which they move. If this solid be of other than sufficiently perfect con- 

 ducting material, of wood and glass, or of metal and glass, for instance, as in 

 the instriiment ordinarily made, it is quite imperfect and indefinite in its 

 indications, and is not worthy of being even called an electroscope, as it may 

 exhibit a divergence when the difference of potentials which the operator 

 desires to discover is absolutely zero. It is interesting to remark that 

 Faraday first remedied this defect by coating the interior of the glass case 

 with tinfoil cut away to leave apertures proper and sufficient to allow indi- 

 cations to be seen, but not enough to cause these indications to differ sensibly 

 from what they would be if the conducting envelope were completely closed 

 around it ; and that not till a long time after did any other naturalist, mathe- 

 matician, or instrument-maker seem to have noticed the defect, or even to have 

 unconsciously remedied it. 



§ 3. Electrometers may be classified in genera and species according to the 

 shape and kinematic relations of their parts ; birt as in plants and animals a 

 perfect continuity of intermediate species has been imagined between the 

 rudimentary plant and the most perfect animal, so in electrometers we may 

 actually construct species having intermediate qualities continuous between 

 the most widely different genera. But, notwithstanding, some such classifi- 

 cation as the following is convenient with reference to the several instruments 

 commonly in use and now to be described : — 



I. Repulsion electrometers. 



Pair of diverging straws as used by Beccaria, Yolta, and others, last 

 century. 



Pair of diverging gold leaves (Bennet). 



Peltier's electrometer. 



Dclmann's electrometer. 



Old-station electrometer, described in lecture to the Eoyal Institu- 

 tion, May 1860 ; also in Nichol's Cyclopredia, article " Elec- 

 tricity, Atmospheric" (edition 1860), and in Dr, Everett's 



