Electromotive Forces in the Voltare Cell. 347 
21. To see if the actual behaviour of such cells at all bears 
out a hypothesis formed on the above plan, I have made some 
rough experiments on the lines suggested ; that is, I passed 
the same current through different simple cells. There are so 
many sources of uncertainty and of variation, that it would 
be very difficult to get really definite and reliable results. 
Thus, for instance, the back H.M.F. will depend considerably 
upon how long the current has been flowing, and so the read- 
ings will differ according to the time they are taken. The 
metals I used were zinc-zinc, zinc-copper, and zinc-platinum ; 
and it was found necessary to put the cathode-plate in a porous 
cell to avoid deposition of zinc on it. But it was now diffi- 
cult to compare the cells easily when arranged in series, 
because different porous pots had different resistances. I 
therefore ultimately decided to use the same porous pot and 
the same anode zinc plate, and to substitute 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 380,000 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 3 ohm) coil placed in the 
circuit. 
Any two values of the strength of current enabled the in- 
ternal resistance of the cell to be calculated, provided its 
E.M.F. remained constant. With low currents it did seem to 
be fairly constant, and a mean value of the internal resistance 
ry is reckoned from these as } chm. 
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 H.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 
