152 Dr. J. Henderson on Cadmium Standard Cells. 



have its lowest values. Assuming these results, interpret them 

 in the language of the undulatory theory of light." 



If we suppose that R = 0, 



1 i _ P_ + °" Jt 



E^E^-n 2 ' 

 and 



r — -2 — X ' 9 9 > 



V" p pr — n* 



if v be the velocity corresponding to cr=0. 



XII. Cadmium Standard Cells. By John Henderson, D.Sc, 

 A.I.E.E., .Head of the Physical Department, Borough Poly- 

 technic Institute, London, S.E* 



THE following paper contains an account of the results 

 of experiments on a special form of cadmium cell, 

 which were made by the author in order to clear up several 

 points of considerable importance on which information was 

 necessary if the cell is to be employed as a standard of electro- 

 motive force. 



The points specially investigated in this research were the 

 following : — 



(1) To see how far reliance might be placed in cadmium 

 cells constructed with chemicals ordinarily described as 

 " pure," but not specially purified. 



(2) To try the effect of constructing the cell, using moist 

 crystals of (JdS0 4 in place of a saturated solution of CdS0 4 f. 



(3) To investigate the effect on the E.M.F. of employing 

 " acid " instead of neutral salts. 



(4) To measure the temperature-coefficient. 



(5) To investigate the nature of the time lag of E.M.F. 



(6) To test the rate of recovery of the cells from polarization. 

 On account of the large temperature -coefficient of the 



Clark cell and the uncertainty as to the exact value of its 

 E.M.F., it was decided not to use it as the standard of 

 E.M.F. in the following measurements, but to compare the 

 various cells with one cadmium cell, the value of the E.M.F. 

 of which, preliminary experiments had shown to have settled 

 down to a constant value. This cell was always kept at the 

 same temperature throughout all the experiments, its E.M.F. 



* Communicated by the Author. 



+ When this research was commenced, owing to the absence of any 

 published description, the author was unaware that crystal cells had been 

 constructed previously by Prof. Callendar. 



