Units in the Electromagnetic Unit of E.M.F. 303 



an application of PoggendorfF's method of compensation, 

 in which, by applying the poles of a gravity Daniell to two 

 such points in the circuit of two cells of the same kind that 

 no current was caused to flow through the derived circuit in 

 which the first cell was thus placed, the E.M.F. of the two 

 compensating cells was determined in terms of that of the 

 compensated cell, the resistance between its poles, and the 

 total resistance in the circuit of the former. By using two 

 values of these resistances, and assuming that the electromo- 

 tive forces were the same in the two cases, two equations were 

 obtained, from which the E.M.F. and resistance of the com- 

 pensating-battery could be calculated. The result showed that, 

 with a total resistance of about 10 ohms in circuit, the E.M.F. 

 of each of the compensating-cells was diminished by only about 

 *6 or '7 per cent, after the current had flowed for several 

 minutes. The internal resistance of the two cells deduced 

 from these experiments agreed within about 1 per cent, with 

 that calculated in the ordinary way from the readings of a 

 quadrant-electrometer when the battery-poles were applied to 

 it (1) insulated, (2) connected by a resistance of 1*96 ohm. 



It should be stated that Mr. Shida' s later determinations of 

 v were made exclusively with Thomson's tray Daniells. The 

 plates of these cells are about 2 feet square, and the resistance 

 of each from -04 to *1 ohm. Mr. Shida found no trace of pola- 

 rization at all in these cells with the resistances he used in 

 circuit, 



How Dr. Wright could suppose for a moment that any 

 person who had a knowledge of Ohm's law could confound 

 the difference of potentials at the two ends of the external 

 resistance with the electromotive force of the cell I cannot 

 imagine ; yet it is on this supposition that his long note in the 

 July number of the Philosophical Magazine, with its allega- 

 tion of a 2-per-cent. error in Mr. Shida's result, is founded. 

 Surely no " explicit statement " on the part of any one was 

 required that one of the commonest rules of electrodynamics 

 was observed in the calculations ! 



The discrepancies, however, which Dr. Wright points out in 

 the numbers which Mr. Shida has given in his paper are real, 

 but admit of easy explanation. I find, on referring to a report 

 of work done last summer furnished by Mr. Shida to Mr. J. T. 

 Bottomley, and dated 25th July 1880, which includes an account 



vegetable parchment, contained in a large shallow glass vessel filled up 

 with a solution of zinc sulphate. Crystals of copper sulphate are placed 

 in a glazed earthenware vessel resting on the zinc, and communicating by 

 a glazed earthenware tube with the liquid immediately above the copper 

 plate. 



