296 Tlie Clark Cell when Producing a Current. 



the value of — (E— e). In the present state of our informa- 

 tion we are not entitled to make any assumption whatever on 

 this question. 



On page 277 Mr. Skinner gives a curve showing the 

 behaviour of a Clark cell when continuously short-circuited 

 so as to yield about '01 ampere. The drop of P.D. is shown 

 in this curve to increase continuously. From this it appears 

 to be inferred (conclusion "6", page 278) that u the electro- 

 motive force of polarization slowly increases when the current 

 is maintained;" and further on (page 279) Mr. Skinner says: — 

 " In the experiments of Threlfall the sign of the term de- 

 pending on time was found to be negative. In some of my 

 earlier experiments it appeared to be negative, but this was 

 traced to irregularity in the working of the compensator; and 

 the effect has always been positive since the Clark cells have 

 been used in the place of the Leclanche." 



Now Mr. Pollock and I specially guarded ourselves in the 

 paper referred to from any statement whatever as to the 

 u electromotive force of polarization," having the fear of 

 Lord Bayleigh and Professor Ayrton very properly before 

 our eyes. Secondly, w T e fortunately have all our notes intact, 

 and find that we kept a much better watch on the compen- 

 sator than Mr. Skinner appears to have done ; and there is 

 no doubt whatever that, in the case we examined, the P.D. 

 drop diminished with time, and this was to be explained 

 neither by change of compensator nor by heating of short- 

 circuiting resistance. We cannot ask the Editors to afford 

 space for the voluminous numerical evidence to exhibit this 

 rather unimportant fact ; but we satisfied ourselves on this 

 point in 1888, and since Mr. Skinner's paper appeared have 

 gone into the matter again and find it to be as we stated in 

 our paper. As a matter of fact our cells were not made up 

 quite in the same way as Mr. Skinner's, and the zincs were 

 wrapped in parchment-paper, the resistance of which may, 

 not improbably, have varied considerably. In instituting his 

 comparison Mr. Skinner apparently overlooked this point, 

 and this leads me to think that he did not refer to our paper 

 very carefully, though probably as carefully as it deserved. 

 We are the more inclined to this view from the statement 

 appearing in §8, "Conclusion/'' page 278, that "From the 

 magnitudes of the quantities found in these experiments, it 

 follows that small currents of approximately known value 

 can be obtained by the use of large Clark cells of small 

 internal resistance, which may be neglected in comparison 

 with the large external resistance." Seeing that the primary 

 object of our work was to discover whether such a proposition 



