60 Messrs. R. T. Glazebrook and S. Skinner. [Mai. 17, 



It has already appeared that experiment gives for I in No. 1 

 2-3 x 10 6 , and in No. 2 2*65 x 10 6 . The difference is probably dne to 

 error in estimating the lead of the brushes, which is difficult, owing 

 to uncertainty in the position of the neutral line on open circuit. 



II. " On the Clark Cell as a Standard of Electromotive Force." 

 By R. T. Glazebrook, M.A., F.R.S., Fellow of Trinity 

 College, and S. Skinner, M.A., Christ's College, Demon- 

 strator in the Cavendish Laboratory, Cambridge. Received 

 February 17, 1892. 



(Abstract.) 

 The paper consists of two parts : — 



In Part I an account is given of experiments on the absolute 

 electromotive force of a Clark cell. 



This was determined in the manner described by Lord Rayleigh 

 (' Phil. Trans.,' 1884) in terms of a known resistance and the electro- 

 chemical equivalent of silver. 



The resistance used was a strip of platinoid about 1 cm. wide and 

 0*05 cm. thick wound on an open frame. It was immersed in a bath 

 of paraffin oil, and the currents used, varying from about 075 to 

 rather over 1*4 amperes, did not raise its temperature sufficiently to 

 affect the result. It had a resistance of nearly 1 B.A. unit. This 

 was determined in terms of the original B.A. units. As part of the 

 object of the experiments was to test the memorandum on the use of 

 the silver voltameter recently issued by the Electrical Standards 

 Committee of the Board of Trade, the large currents mentioned 

 above were purposely employed. The silver voltameters were treated 

 in accordance with the instructions in the memorandum. 



The standard cell to which the results are referred is one con- 

 structed by Lord Rayleigh in 1883, probably No. 4 of the cells 

 described in his paper already quoted. 



The results have been reduced on the supposition that 1 B.A. 

 unit is equal to 0*9866 ohm ; if we take the number 0*9535* as 

 representing the value in B.A. units of the resistance of a column 

 of mercury at 0°, 1 metre long, 1 sq. mm. in section, the above is 

 equivalent to saying that the length of the mercury column having 

 a resistance of 1 ohm is 106*3 cm. It has also been assumed that 

 the mass of silver deposited in one second by a current of 1 ampere 

 is 0"001118 gramme, and that the coefficient of change of B.M.F. with 

 temperature of a Clark's cell is 0*00076. This last result has been 

 verified by us in Part II. 



* This number is the mean of the best recent results. 



