516 



11EP0RT — 1884. 



the other plates one after the other, making the current as nearly the 

 same each time as convenient (by adjusting resistance) and allowing for 

 outstanding discrepancies. An amperemeter placed in circuit measured the 

 current, and the voltameter used was a reflecting galvanometer with some 

 30,000 ohms in its circuit. Its indications were interpreted absolutely by 

 tapping off, at the same time as the cells, the difference of potential between 

 the terminals of an ohm (or j or ^ ohm) coil placed in the circuit. 



Any two values of the strength of current enabled the internal 

 resistance of the cell to be calculated, provided its B.M.F. remained con- 

 stant. With low currents it did seem to be fairly constant, and a mean 

 value of the internal resistance r is reckoned from these as 1- ohm. 



The area of each plate under the liquid was exactly the same, and 

 measured 3 inches by 2| inches. Both faces of each plate were exposed, 

 though naturally one face was more active than the other. 



The arithmetical reductions are rather long ; the results are all that I 

 give. It will be perceived they are anomalous in places, a great deal of 

 this being dependent on whether the reading of E.M.F. was taken soon 

 after a current- change or not. As I said before, the plan of experiment is 

 avowedly rough, though the actual readings were carefully taken ; but 

 without understanding more about the circumstances of the case, and 

 what possibilities of variation there are, I do not see how to plan a 

 perfect system of experiment on the subject. 



I will first give relative numbers, simply comparing the differences of 

 potential between the terminals of the three cells when the same current 

 is going through each, the resistance of each being the same, viz. 

 ^ ohm ; and then I will interpret the observations absolutely, calculating 

 the E.M.F. of the cell under different currents, and seeing what empirical 

 formula will best fit it. 



Relative differences of potential "between the terminals of three voltameters of 

 ihe same resistance, through each of which the same current is driven by 

 an auxiliary battery. Anode of each cell, zinc; cathode, zinc-copper and 

 platinum respectively . 



In the above table the difference of potential between the terminals is 

 written negative • when it opposes the current, and positive when it helps 

 it on. 



We will now interpret similar measurements absolutely, reckoning 

 the actual E.M.F. of each cell, and try to fit an empirical formula to it on 

 the plan of those (in sect. 20) already guessed; assuming /(C) a linear 



