and on the Construction of Voltaic Batteries. 71 



utmost care can scarcely provide, it is but to be expected, so 

 long as the same materials are used, that the greatest contra- 

 riety of opinion will exist on points apparently so easy to de- 

 termine. A method of avoiding those errors which have their 

 origin in the irregularity of the mutual action of the acid and 

 zinc will be submitted to you towards the end of my letter. 



In order to avoid a too frequent recurrence to description 

 in the course of this paper, I will, at this point, state generally 

 some of the precautions (beyond the common and more obvious 

 ones) that were taken to ensure accuracy in the results of the 

 experiments. 



The magnetic needle was never resorted to as a measurer of 

 the amount of action or of the quantity of electricity developed; 

 but this was estimated by what appears to me to be the more 

 certain, though infinitely more laborious method, of finding, 

 by the balance, the equivalent of zinc expended, or by actually 

 measuring the volume of the evolved hydrogen. In no instance 

 was the one calculated merely from the volume or weight of 

 the other. In general both the balance and the meter were 

 employed, whatever the number of the series in any experi- 

 ment to be examined, and the number amounted in some cases 

 to as many as fifty. I had anticipated that some curious re- 

 sults would appear from this mode of testing the phaenomena, 

 and have not been altogether disappointed. Again, when plates 

 of amalgamated zinc were used in comparative experiments, 

 in which equal ones, or differing by a certain ratio, were wanted, 

 these comparative values were not estimated by the mere 

 measure of their surfaces, but, by actually finding the amount 

 of action upon them in a given time, by previous immersion 

 in acid of a kind similar to that intended to be used ; for an 

 equality in the extent of surface does not ensure an equality 

 of voltaic action, nor does that action increase in the same ratio 

 as the surface may be increased, as has hitherto been believed. 

 Again, the zinc employed throughout these experiments was 

 always of the same quality, indeed was cut from the same sheet, 

 and itsequivalent determined, with hydrogen as unity, and found 

 to be S4--5. And in order, as far as possible, to reduce the num- 

 ber of the conditions involved in these experiments and requi- 

 ring to be attended to and estimated, one of such was entirely 

 avoided, viz. that of variation in the distance between the two 

 elementary plates. Whether used in independent or compa- 

 rative experiments the mass of fluid interposed between the 

 zinc and copper plates was invariably equal to one inch. The 

 acid employed was the diluted sulphuric, and in those propor- 

 tions which have become standard ones, through their having 

 been used as such by yourself and Dr. Faraday. It is, per- 

 haps, almost needless to remark that in any case where local 



