Electromotive Force of different Forms of the Clark Cell. 117 



On the Variation of the Electromotive Force of different 

 Forms of the Clark Standard Cell with Temperature and 

 with Strength of Solution." By H. L. Callendar, M.A., 

 F.R.S., McDonald Professor of Physics, and H. T. BARNES, 

 M.A.Sc, Demonstrator of Physics, McG-ill University, 

 Montreal. Received August 12, 1897. 



§ 1. Objects of the Investigation. 



The primary object of the present series of experiments was that 

 of equipping the McDonald Physics Building of McGill University 

 with a reliable and accurate set of standard cells, and not that of 

 forming the subject of a communication to any scientific paper. In 

 the course of the work, however, several points have come under our 

 notice, which we venture to think may be of interest to others 

 engaged in any investigation requiring the employment or construc- 

 tion of such standards. 



Among other points, we have devoted special attention to the 

 accurate determination of the temperature- coefficients of various 

 forms of Clark cell ; to the construction of cells free from " diffu- 

 sion-lag " consequent upon change of temperature ; and to the inves- 

 tigation of the limits of accuracy attainable with Clark cells under 

 both constant and varying temperature conditions. 



We have succeeded in making a very simple modification in the 

 Board of Trade form of Clark cell, which makes it equal to any 

 other form in respect of freedom from diffusion-lag ; and we have 

 made several forms of cell hermetically sealed with glass and 

 platinum, which we hope will stand the test of time better than 

 those sealed with wax and marine glue. 



We have also made a special investigation of the effect of changes 

 of strength of the solution of zinc sulphate on the E.M.F., involving 

 determinations of the solubility of zinc sulphate and of the density 

 of the solutions, which appear to lead to very simple formulae of 

 some theoretical interest. 



§ 2. Preliminary Work. 



At the outset of our work, we were extremely fortunate in finding 

 the laboratory already equipped by the liberality of Mr. W. C. 

 McDonald, the donor of the Physics Building, with a very fine and 

 complete set of resistance standards and electrical measuring in- 

 struments, collected by Professor John Cox, under whose supervision 

 the laboratory was planned and erected. There were in the collec- 

 tion several portable Clark cells by Muirhead, and a set of Carhart 

 cells by Queen and Co. were shortly added. 



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