MK. F. E. SMITH ON THE NORMAL WE8TON CAI'MII'M CELL. 411 



The Temperature Coefficient and " Lag." 



In experimental work involving the use of the Clark cell, temperature corrections 

 have invariably to be introduced owing to the high value of the temperature 

 coefficient. This is the most serious objection to its use. The temperature coefficient 

 of the cadmium cell is much smaller and has been determined by JAEGER and KAHLE, 

 who give the following equation connecting temperature and E.M.F. : 



E, = 1-0186-0-000038 (-20) -0'00000065 (t- 



At the National Physical Laboratory six cells were chosen and their temperatures 

 were varied very slowly from 10 C. to 30 C. The maximum rate of change of 

 temperature was 1 C. per hour, and before making an observation at any particular 

 temperature the oil bath in which the cells were immersed was kept at that 

 temperature for at least an hour, a toluene thermostat capable of maintaining a 

 constant temperature to 0'01 C. being employed. The cycle of temperature was 

 repeated three times. The agreement between the cells was excellent, and the mean 

 values of the E.M.F.'s were taken to obtain the temperature coefficient by the 

 method of least squares. The resulting temperature formula is 



E, = E 17 -0-000034 6 (*-17) - -00000066 (- 17)*. 



This is in very good agreement with JAEGER and KAULE'S formula. The changes 

 in E.M.F. from 10 C. to 15 C., 10 C. to 20 C., and 10 C. to 30 C., as deduced 

 from the two formulae, are given below. 



" C. JAKGER and KAHLE. N.P.L. 



10 to 15 .' . . . ', 0-00015, volt. -00014, volt. 



10 20 .;;-/ . .- 0-00032, 0-00031 



10 30 . -. ; . :-.--*- 0'00077 0-00076, 



The lag of E.M.F. with respect to temperature changes was shown by AYRTON and 

 COOPER* to he much greater in the Board of Trade tube form of cell than in the 

 H form. They concluded that there is a " simple lag," which may be removed by a 

 comparatively short interval of constant temperature, and a " semi-permanent lag," 

 which requires many hours of steady temperature for its complete removal. We have 

 made similar observations on the Weston cadmium cell and find evidence of the same 

 lag in it. The effect is, however, very small when the temperature changes are slow 

 and the range of temperature only a few degrees, as in the experiments of AYRTON 

 and COOPER on the Clark cell. When the range of temperature is about 1 5 C. and 

 the change of temperature very rapid, a difference in E.M.F. of about 30 microvolts is 

 often observed after the normal temperature of the cell has been restored for 4 or 



* AYRTON and COUPKIE, ' Key. Soc. Proc-.,' 59, p. 368, 1896. 

 8 O 2 



