MR. F. K. SMITH (IX Tin: N T ORMAL WTvSTON CADMIUM CELL. 415 



Electrical Standards Department. That a small current may be taken from the cell 

 without any permanent effect is proved from the olwervations when cells have been 

 sln>rt-niriiitf<l. hut the ell'ert <>f :i comparatively large charging current may be more 

 serious. Lord I! \YI.KHMI attempted to manufacture a Clark cell by the formation of 

 electrolytic mercurous sulphate inside an H vessel, the anode being mercury, the 

 electrolyte /.inc sulphate, and the cathode an amalgam of zinc. The cells so formed 

 were not constant, and their E.M.F.'s were low. In 1904 we attempted in a similar 

 way to produce cadmium cells, hut it was evident that normal mercurous sulphate was 

 not formed, as the resulting salt was highly coloured ; it was sometimes yellow, but 

 more often green. The fact that the depolariser in some of our anomalous cells has, 

 after a long period, turned a yellowish-green suggested to us that its formation might 

 he accelerated by small charging currents. The constancy of other cells subject to 

 the same treatment is certainly against such a view, hut a slight difference in the 

 original composition of the depolarisers might account for the more rapid change. 

 To test this point, we placed a normal cell in circuit with, but in opposition to, two 

 Leclanch6 cells for 18 hours. At the end of that time a green compound had formed 

 between the mercury electrode and the glass, but the depolariser appeared to be 

 unchanged. There is little doubt, however, but that some of the green salt was 

 present over the whole surface of the mercury. The E.M.F. of this cell was at first 

 very high, but in 4 weeks it gradually fell to 1 '01 833 volts. The observations which 

 we have so far made do not enable us to say whether any further fall is probable, 

 but it is evident that the small charging currents to which a cell is subjected in a 

 potentiometer circuit do not seriously affect its E.M.F. 



Portability of tlie Cell. 



Many of the cells made at the National Physical Laboratory are portable, and may 

 be sent through the post. In these cells the two limbs of the H .vessel are constricted 

 at points about 1^- centims. from their lower ends, and when making up the cell, 

 cadmium sulphate crystals are added until the upper surface of a crystalline layer is 

 on a level with the narrowest part of the tube in which the crystals are placed. 

 Cadmium sulphate solution is then added and the cells are exposed in a warm room 

 for a week or more before sealing. Some of the liquid evaporates, and many of the 

 fine crystals are loosely cemented together. This crystalline plug keeps the contents 

 in their proper places and enables the cell to be inverted. 



Conclusions. 



(1) Tli.- electromotive force of the Weston cadmium cell is the same whether it 

 contains electrolytic mercurous sulphate, chemically prepared sulphate, the salt as 

 precipitated by the dilution of hot strong sulphuric acid in which mercurous sulphate 

 is dissolved, or that resulting from the action of fuming sulphuric acid on mercury. 



