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February 23, 1860. 



Sir BENJAMIN C. BROUIE, Bart., President, in the Chair. 

 The following communications were read : 



I. " Measurement of the Electrostatic Force produced by a 

 Daniell's Battery." By Professor WILLIAM THOMSON, 

 F.ft.S. Received January 21, 1860. 



In a pnper " On Transient Electric Currents," published in the 

 Philosophical Magazine for June 1853, I described a method for 

 measuring differences of electric potential in absolute electrostatic 

 units, which seemed to me the best adapted for obtaining accurate 

 results. The "absolute electrometer" which I exhibited to the British 

 Association on the occasion of its meeting at Glasgow in 1855, was 

 constructed for the purpose of putting this method into practice, 

 and, as I then explained, was adapted to reduce the indications of 

 an electroscopic * or of a torsion electrometer to absolute measure. 



The want of sufficiently constant and accurate instruments of the 

 latter class has long delayed my carrying out of the plans then set 

 forth. Efforts which I have made to produce electrometers to 

 fulfil certain conditions of sensibility, convenience, and constancy, 

 for various objects, especially the electrostatic measurement of gal- 

 vanic forces, and of the differences of potential required to produce 

 sparks in air, under definite conditions, and the observation of natural 

 atmospheric electricity, have enabled me now to make a beginning of 

 absolute determinations, which I hope to be able to carry out soon in 

 a much more accurate manner. In the meantime I shall give a slight 

 description of the chief instruments and processes followed, and state 

 the approximate results already obtained, as these may be made the 



* I Lave used the expression " electroscopic electrometer," to designate an 

 electrometer of which the indications are merely read off in each instance by a 

 single observation, without the necessity of applying any experimental process of 

 weighing, or of balancing by torsion, or of otherwise modifying the conditions 

 exhibited. 



VOL. X. 2 A 



