514 Sir W. Thomson on the Determination of 



The greatest values of this will be found at the curved bound- 

 ing edge, for which r == sec <p. Thus we find 



\ i+ d? + ?aw*y \ 



{ =-0948, and therefore inclination = 5° 25' at E H [ h 

 \ = -1892, „ „ „ = 10° 43' at B J ) 



Hence we see that the inaccuracy due to neglecting the 

 square of the tangent of the inclination in the mathematical 

 work cannot be large. The exact value of the inclination at 

 E is tan- 1 (- ^2) -120°, or 5° 16', which is less by 9' than 

 its value by (16). 



LXIV. On the Application of the Deci-ampere or the Centi- 

 ampere Balance to the Determination of the Electromotive 

 Forces of Voltaic Cells. By Sir William Thomson, F.R.S,* 



THE method described in this paper for the determination, 

 in absolute measure, of the electromotive forces of vol- 

 taic cells, consists in the use of one of my standard ampere- 

 balances instead of the tangent-galvanometer in the method 

 given in the following statement, which I quote from Kohl- 

 rausch's < Physical Measurements,' pp. 223, 224, 230:— " The 

 only methods applicable to inconstant elements, of which the 

 electromotive force varies with the current-strength, is to bring 

 the current to zero by opposing an equal electromotive force. 

 Poggendorff's method, which is very convenient, as it involves 

 no measurement of internal resistance, requires the use of a 

 galvanoscope, Gr, a galvanometer, T, and a rheostat, R, and, 

 in addition, that of an auxiliary battery, S, of constant electro- 

 motive force, greater than either of those which are to be 

 compared. The arrangement of the experiment is shown in 

 the figure. In the left division of the G T 



circuit are the galvanoscope G, and 

 the electromotive force E to be mea- 

 sured ; in the right, the auxiliary 

 battery S and the galvanometer T. 



E and S are so placed that their ^ — \\ -^ — lj ' 



similar poles are turned towards each E . s 



other. In the middle part of the cir- %' 



cuit, which is common to both batteries, is the rheostat R. 

 " As much rheostat resistance W must now be intercalated 



* Communicated by the Author, being a paper read at the Manchester 

 M eeting of the British Association, with additions. 



K 



