ON STANDARDS 01' ELECTRICAL RESISTANCE. 507 



reverse effect, the earth-reading becoming smaller as the instrument is 

 warmed, or larger when it is cooled. I have ascertained that these changes 

 are not due to changes in the electric capacities of the Ley den jars ; and I 

 have found that the change, if any, of specific inductive capacity of glass by 

 change of temperature is excessively small, in comparison to what would be 

 required to account for the temperature errors of these instruments, which 

 probably must be due to thermo-elastic properties of the platinum wire, or 

 of the stretching-springs, or of the aluminium balance-lever, or to a com- 

 bination of the effects depending on such properties ; but I have endeavoured 

 in vain, for several years, and made many experiments, to discover the precise 

 cause. It surely will be found, and means invented for remedying the error, 

 now when I have an instrument in which the error is in the opposite di- 

 rection to that of most of the other instruments. It is of course mi;ch 

 greater in some instruments than in others : in some it is so great that the 

 earth-reading is varied by as much as twenty divisions by the warmth of the 

 hand in the course of five or ten minutes after commencing to use the in- 

 strument, if it has been previoiisly for some time iu a cold place. Its in- 

 fluence may be eliminated, not quite rigorously, but nearly enough so for most 

 practical jiurposes, by frequently taking earth-readings (§ 30) and proceeding 

 according to the directions of § 31. 



(4) A fourth fault in the ijortable electrometer is, that the diameter of 

 the guard-plate and upper attracting disk, which ought to be infinite, are not 

 sufiiciently great, in proportion to the greatest distance between them, to 

 render the scale quite uniform in its electric value throughout. A careful 

 observer will, however, remedy the greater part of the error due to this 

 defect, by measuring experimentally the relative (or absolute) values of the 

 scale-division in different parts of the range. There will, however, remain 

 uncorrected some irregularity, due to influence of the distribution of elec- 

 tricity over the uncoated inner surface, in the instruments as hitherto made, 

 in all of which the inner surface of the jar is coated with tinfoil only below 

 the guard-plate, so that the uj^per surface of the gTiard-jjlate may be seen 

 clearlj-, in order that the observer may always see that all is in order about 

 the ahimiuium square and aperture round it ; and particularly that there 

 arc no injurious shreds or minute fibres. But the irregular influence of the 

 electrification of the uncoated glass, if foimd sensible, will be rendered insen- 

 sible l)y continuing the tinfoil coating an inch above the upper surface of 

 the guard-plate. 



§ 33. All faults, except the temperature error, depend on the smallness of 

 the instrument ; and if the observer chooses to regard as portable an instru- 

 ment of thirty centimetres (or a foot) diameter, with all other dimensions, 

 and all details of construction, the same as those of the instrument described 

 above, he may have a portable electrometer practically free from three of the 

 four faults described. But it is scarcely to be expected that a small instru- 

 ment (12i centimetres high, and 8| centimetres in diameter) which may be 

 carried about in the pocket can be free from such errors. They are, 

 however, so far remedied as to be probably not perceptible in the large sta- 

 tionary instrument which I now proceed to describe. 



Standard Electeometee. 



§ 34. This instrument (figs. 12, 13,& 14, Plate VI.) differs from the portable 

 electrometer only in dimensions, and in certain mechanical details, which are 

 arranged to give greater accuracy by taking advantage of freedom from the exi- 

 gencies of a small portable instrument. It is at present called the standard 



