41(5 ME. F. E. SMITH ON THE NORMAL WESTON CADMIUM CELL. 



(2) The size of the crystals of mercurous sulphate prepared by the first two and 

 the last of the above methods usually varies from 5 microns to 15 microns, and we 

 have found no electromotive difference between a saturated solution of these crystals 

 in a cadmium sulphate solution and a saturated solution of very large mercurous 

 sulphate crystals in the same solvent. 



(3) The simplest method of preparing mercurous sulphate is by chemical precipita- 

 tion, and the product is more uniform than that of any other method so far examined. 



(4) The electromotive force is the same whether the cadmium amalgam is prepared 

 by the electro-deposition of cadmium in mercury, or by the fusion of cadmium and 

 mercury. At normal working temperatures either a 10 or a 12 per cent, amalgam 

 may be used. 



(5) The electromotive force is probably constant over long periods of time, but the 

 cells should be compared with those of a standardising institution every 12 months ; 

 failing this, they should be compared with freshly set up cells. 



(6) The change of E.M.F. with temperature may be calculated from JAEGER and 

 KAHLE'S equation or from that obtained at the National Physical Laboratory. As 

 the former has been used for so many years, we suggest its universal adoption. 



(7) The small charging and discharging currents to which a cell is subjected in a 

 potentiometer circuit do not seriously affect the value of the electromotive force. 





 We desire to express our thanks to the Committee of the British Association for 



grants of money for the purchase of materials ; to Dr. GLAZEBROOK for much advice 

 concerning the construction of the cells, and to Mr. J. A. SADD, of the Central 

 Technical College, and Mr. TINSLEY, for constructing standard cells to compare 

 with ours. 



APPENDIX. 



Added December 4, 1907. 



ON THE COMPARISON OF THE ELECTROMOTIVE FORCES OF WESTON CADMIUM CELLS 

 PREPARED AT WASHINGTON, AT PARIS, AT BERLIN, AND AT TfiDDINGTON. 



Dr. BURGESS of the National Bureau of Standards, Washington, journeyed to 

 Paris and Berlin after his visit to Teddington, and very kindly took with him a 

 number of cadmium cells from the National Physical Laboratory in addition to 

 others from Washington. Dr. F. A. WOLFF has forwarded us a report on the 

 measurements of the cells in Paris, from which we make the following abstracts. 



