278 Dr. 0. R. A. "Wright on the Determination of 



again in reversed order; so that, by taking the averages of the 

 two sets, any error due to running down of the electrometer 

 during the readings might be eliminated (in practice the run- 

 ning down during the period was insensible, the variation 

 being usually at most only 2 or 3 per cent, during the whole 

 day, and often much less). When three pairs of plates were 

 used, the three zincs were connected respectively with cups 

 1, 3, and 5 ; whilst cup No. 2 was connected with No. 4, 

 and also, by means of a movable wire, with either of three 

 other cups, Nos. 10, 11, and 12, with which respectively the 

 three copper plates were connected : so that when No. 2 was 

 connected with 10, as represented in the figure, the com- 

 binations GiZ i} Ciz.i, and c^ 3 could be read off by connecting 

 the double switch with 1 and 2, 2 and 3 (or 3 and 4), and 4 

 and 5 successively; and similarly for the other combinations. 



107. The ultimate results of upwards of a hundred series of 

 valuations of the electromotive forces of various combinations, 

 mostly lasting over four hours, were as follows: — 



(1) With the stronger solutions used (specific gravities 1*100 

 to 1*175) the E.M.F. set up after the first few minutes remained 

 sensibly constant for several hours (the temperature being 

 constant), never differing from the final average of the four 

 average sets of readings made in each of the first four hours 

 by amounts outside the limits of observational error. For 

 instance, the following values were obtained in one experi- 

 ment, in which the temperature throughout was close to 18°, 

 the specific gravity of the solutions being 1*175: — 



Combination. 



Average E. M.F. determined during the 



Final 



average. 



1st hour. 2nd hour. 



3rd hour. 



4th hour. 



Aina^aicated zinc — "1 

 e^ctro-copper . . . J 



Bright zinc — electro- \ 

 copper J 



Amalgamated zinc — "1 



bright copper . . . J 



Bright zinc — bright 1 



1-112 

 1-110 

 1-119 

 1-117 



1-113 

 1-109 

 1-122 

 1-115 



1-114 

 1-112 

 1-122 

 1-118 



1-114 

 1-110 

 1-120 

 1-118 



1-1132 

 1-1102 



1-1207 

 1-1170 









Very similar results were obtained in all the other cases. 

 After twenty-four hours the E.M.F. usually diminished to a 

 greater or less extent. These changes are referred to later on 

 (§ 108), being probably due to oxidation of the metals by dis- 

 solved air. 



(2) With weaker solutions (sp. gr. 1*0065 to 1*050) the 

 E.M.F developed during the first half hour or so ..as usually 



