326 



according to my thermo-dynaruie estimate. This is as good an agree- 

 ment as could be expected when the difficulties of the investigations, 

 and the uncertainty which still exists as to the true measure of the 

 electromotive force of the Daniell's element are considered. It must 

 indeed be remarked that the electromotive force of Daniell's battery 

 varies by two or three or more per cent, with variations of the solu- 

 tions used ; that it varies also very sensibly with temperature ; and 

 that it seems also to be dependent, to some extent, on circumstances 

 not hitherto elucidated. A thorough examination of the electro- 

 motive force of Daniell's and other forms of galvanic battery, is an 

 object of high importance, which it is to be hoped will soon be at- 

 tained. Until this has been done, at least for Daniell's battery, the 

 results of the preceding paper may be regarded as having about as 

 much accuracy as is desirable. 



I may state therefore, in conclusion, that the average electromotive 

 force per cell of the Daniell's batteries which I have used, produces a 

 diiference of potentials* amounting to '0021 in British electrostatic 

 measure. This statement is perfectly equivalent to the following in 

 more familiar terms : 



One thousand cells of Daniell's battery, with its two poles connected 

 by wires with two parallel plates of metal y^th of a foot apart and 

 each a square foot in area, produces an electrical attraction equal to 

 the weight of 55 grains. 



II. " Measurement of the Electromotive Force required to pro- 

 duce a Spark in Air between parallel metal plates at dif- 

 ferent distances." By Professor W. THOMSON, F.R.S. 

 Received January 26, 1860. 



The electrometers used in this investigation were the absolute 

 electrometer and the portable electrometer described in my last 

 communication to the Royal Society, and the operations were ex- 

 ecuted by the same gentlemen, Mr. Smith and Mr. Ferguson. The 

 conductors between which the sparks passed were two unvarnished 

 plates of a condenser, of which one was moved by a micrometer 

 screw, giving a motion of -^- of an inch per turn, and having its 



* See 10, 11 of Appendix to the following communication. 





