£36 Royal Society .•— 



neighbourhood, will be sufficiently described in another communi* 

 cation to the Royal Society. Slight descriptions of trial instruments 

 of this kind have already been published in the Transactions of the 

 Pontifical Academy of Rome*, and in the second edition of Nichol's 

 Cyclopaedia (article Electricity, Atmospheric), 1860. 



I hope soon to have another electrometer on the same general prin- 

 ciple, but modified from those hitherto made, so as to be more 

 convenient for accurate measurement in terms of constant units. 

 In the meantime I find, that, by exercising sufficient care, I can 

 obtain good measurements by means of the divided ring electro- 

 meter of the form described in Nichol's Cyclopaedia. 



In the ordinary use of the portable electrometer, a considerable 

 charge is communicated to the connected inside coatings of the 

 Leyden phials, and the aluminium index is brought to an accurately 

 marked position by torsion, while the insulated metal case surround- 

 ing it is kept connected with the earth. The square root of the 

 reading of the torsion-head thus obtained measures the potential, to 

 which the inside coatings of the phials have been electrified. If, 

 now, the metal case referred to is disconnected from the earth and 

 put in connexion with a conductor whose potential is to be tested, 

 the square root of the altered reading of the torsion-head required 

 to bring the index to its marked position in the new circumstances 

 measures similarly the difference between this last potential and that 

 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" 69 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 

 themf, the force at a distance of fa of an inch would have been 

 •¥uT» according to the first of the preceding results, or, according to 



* Accadcmia Pontificia dei Nuovi Lyncei, February 1857. 

 f See § 11 of elements of mathematical theory of electricity appended to the 

 communication following this in the ' Proceedings.' 



