322 



with the absolute electrometer according to the process described 

 above, is not sufficiently sensitive to measure directly the electro- 

 static effect of any galvanic battery of fewer than two hundred cells 

 with much accuracy. Not having at the time arrangements for 

 working with a multiple battery of reliable character, I used a second 

 torsion electrometer of a higher degree of sensibility as a medium 

 for comparison, and determined the value of its indications by direct 

 reference to a Darnell's battery of from six to twelve elements in 

 good working order. This electrometer, in which a light aluminium 

 index, suspended by means of a fine glass fibre, kept constantly elec- 

 trified by means of a light platinum wire hanging down from it and 

 dipping into some sulphuric acid in the bottom of a charged Leyden 

 jar, exhibits the effects of electric force due to a difference of poten- 

 tials between two halves of a metallic ring separately insulated in its 

 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 

 * Accademia Pontificia del Nuovi Lyncei, February 1857. 



