[ 295 ] 



XXVII. The Clark Cell when Producing a Current, 



To the Editors of the Philosophical Magazine. 



Gentlemen, 



IN the Philosophical Magazine for September 1894 is a 

 paper by Mr. S. Skinner on " The Clark Cell when Pro- 

 ducing a Current/'' This paper appears to call for one or two 

 remarks from me. Mr. Skinner says (p. 272) : — " Let a cell of 

 electromotive force E and internal resistance R have its poles 

 joined by a wire of resistance r ; then, providing R and r are 

 constant, and there is no polarization, the potential-difference 



between the poles will be -^ •. If, however, there is po- 



. . re 



larization, then this potential-difference will be ^ , where 



1 si + r 



e is the value of the electromotive force required to produce 

 the observed current." 



The value of — (E — e) is the " electromotive force of polari- 

 zation." This quantity is studied by Mr. Skinner, and its 

 evaluation of course involves measurements of R and r. 

 With regard to r, which appears to have been taken from a 

 box for values down to 200 legal obms, and for 147 legal ohms 

 from a german-silver coil in a bath of paraffin oil, we are not 

 given very much information. I would ask, however, whether 

 in the light of Mr. Griffiths' experience (Phil. Trans. A. 

 1893, p. 401), it is safe to assume that the resistance of a coil 

 mounted as described, and carrying a current, can be suffi- 

 ciently ascertained by tests made with only a testing-current. 

 No one ought to be more alive to this than Mr. Skinner, so 

 that I will assume his value of r to have been sufficiently 

 known. With regard to " R," however, the case is different. 

 Its value was measured by Professor Macgregor's method 

 (Trans. Roy. Soc. Canada, iii. 1882, p. 22, referred to as 

 '' by means of the commutator used by Mr. T. C. Fitz- 

 patrick "), in which alternating intermittent currents are fed 

 into a Wheatstone bridge, and commutated for the benefit 

 of a galvanometer. The measurements were not made while 

 the cell under examination was yielding a current for polari- 

 zation observations : consequently the value assigned to 

 — (E — e) rests on the assumption that the resistance of the 

 cell, while yielding a considerable current ('01 ampere) for 

 polarization experiments, was the same as the resistance it 

 exhibited while under test by alternating and intermittent 

 currents. This assumption I consider to be wholly illegiti- 

 mate, and to vitiate every result given by Mr. Skinner for 



X2 



