Chemical Affinity in terms of Electromotive Force. 267 



81. The average current passing during any given experi- 

 ment was determined as follows : — The voltameter having been 

 arranged and the current turned on at a noted time, the differ- 

 ence of potential set up by the current passing between the ends 

 of a known resistance (corrected when necessary for tempera- 

 ture) was determined by means of the quadrant-electrometer; 

 and hence the current passing was known. During the first two 

 or three hours the current usually diminished somewhat, owing 

 to the increasing counter E.M.F. set up in the voltameter; 

 but subsequently it remained sensibly steady, under suitable 

 conditions as to battery-power, temperature, &c. By means 

 of numerous observations the average currents passing in each 

 of various successive periods of time were known ; and hence 

 the average current for the whole time was known. Thus, 

 for example, the average currents in one experiment were as 

 follows : — 



Period. Average current during period. 



Commencement to 6 hours. 

 6 hours to 21 hours 

 21 „ 2\) „ 



Hence the average current for the whole period is 



6 x 22-4+ 15 x 22-1 + 8x21-5 



22*4 micro we bers. 



22-1 



21-5 



29 



= 22-03 microwebers. 



In standardizing the quadrant-electrometer, use was made 

 either of a moderately newly constructed Clark's cell, or of 

 one which had " run down" to a known extent as compared 

 with a newly prepared cell*, the E.M.F. of a moderately new 

 cell being taken from Clark's experiments to be 



l'457{l-(*-15) x 0-0006} volt 



at t° (i. e. the difference of potential set up by the cell between 

 two poles of the same material being expressed by this amount). 

 It hence results that if the value of the B.A. unit of resistance 

 (on which the valuation of the E.M.F. of Clark's cell depends, 



* A large/number of experiments made with Clark's cells showed that 

 whilst the E.M.F. of comparatively newly prepared cells (i. e. a few days 

 to some three months or so old) is sensibly constant when suitable precau- 

 tions are taken in the construction, yet a gradual decrease in the E.M.F. 

 begins to take place after a longer or shorter time and then goes on conti- 

 nuously. Experiments on the construction and the circumstances influ- 

 encing the permanence of these cells and others of different kinds, are in 

 progress, which will be described in a future paper ; it may be noticed, 

 however, that cells prepared with mercurous-sulphate paste, boiled in a 

 Sprengel vacuum and enclosed in hermetically-sealed glass tubes free 

 from air, appear to be preferable to cells prepared in accordance with 

 Clark's directions with a sealing of melted paraffin-wax (Proc. Roy. Soc. 

 xx. p. 444, 1872). 



