323 



of the inside coatings of the phials. Hence the excess of the latter 

 square root above the former expresses in degree and in quality 

 (positive or negative) the required potential. This plan has not 

 only the merit of indicating the quality of the electricity to be tested, 

 which is of great importance in atmospheric observation, but it also 

 affords a much higher degree of sensibility than the instrument has 

 when used as a plain repulsion electrometer ; and, on account 

 of this last-mentioned advantage, it was adopted in the comparisons 

 with the divided ring electrometer. On the other hand, the portable 

 electrometer was used in its least sensitive state, that is to say, with 

 its Leyden phials connected with the earth, when the comparisons 

 with the absolute electrometer were made. 



The general result of the weighings hitherto made, is that when 

 the discs of the absolute electrometer were at a distance of twenty 

 hundredths of an inch, the number of degrees of torsion in the port- 

 able electrometer was 3-229 times the number of grains' weight 

 required to balance the attractive force ; and the number of degrees 

 of torsion was 7' 6 9 times the number of grains' weight found in 

 other series of experiments in which the distance between the discs 

 was thirty hundredths. According to the law of inverse squares of 

 the distances to which the attraction between two parallel discs is 

 subject when a constant difference of potentials is maintained between 

 them *, the force at a distance of -^ of an inch would have been 

 TDT according to the first of the preceding results, or, according to 

 the second, .-g-l^ of the number of degrees of torsion. The mean of 

 these is .-^j, or 1'2 ; and we may consider this number as represent- 

 ing approximately the value in grains' weight at -^ of an inch 

 distance between the discs of the absolute electrometer, correspond- 

 ing to one degree of torsion of the portable electrometer. By com- 

 paring the indications of the portable electrometer with those of 

 the divided ring electrometer, and by evaluating those of the latter 

 in terms of the electromotive force of a Dani ell's battery charged 

 in the usual manner, I find that 284 times the square root of the 

 number of degrees of torsion in the portable electrometer is approxi- 

 mately the number of cells of a Daniell's battery which would pro- 

 duce an electromotive force (or, which is the same thing, a difference 



* See 11 of elements of mathematical theory of electricity appended to the 

 communication following this in the ' Proceedings.' 



