62 



Messrs. R. T. Glazebrook and S. Skinner. [Mar. 17, 



Schuster, Mr. Wilberforce, and ourselves during the past year, all 

 agree closely, the variations among them being rarely greater than 

 about 0-0005 volt. 



The first set of cells, eighteen in number, constructed for the pur- 

 poses of this enquiry were made according to Lord Rayleigh's 

 instructions, using, however, various specimens of the chemicals. 

 These showed some differences at first, but in the course of about two 

 months they had all, with one exception, settled down to close agree- 

 ment with the standard. The exceptional cell has since become 

 normal. In two of these cells mercury was used which had been taken 

 direct from the stock in every-day use in the laboratory. The 

 E.M.F. of these cells was much too low at first, but it gradually 

 increased, and they are now normal. The mercurous sulphate appears 

 to free the mercury from certain harmful impurities. 



Another set of cells were put up, in accordance with the provisional 

 memorandum of the Electrical Standards Committee of the Board of 

 Trade, issued in June last aud quoted below. 



Memorandum on the Preparation oe the Clark's Standard Cell. 



Definition of the Cell. 



The cell consists of mercury and zinc in a saturated solution of zinc sulphate and 

 mercurous sulphate in water, prepared with mercurous sulphate in excess, and is 

 conveniently contained in a cylindrical glass vessel. 



Preparation of the Materials. 



1. The Mercury. — To secure purity it should be first treated with acid in the 

 usual manner, and subsequently distilled in vacuo. 



2. The Zinc. — Take a portion of a rod of pure zinc, solder to one end a piece of 

 copper wire, clean the whole with glass paper, carefully removing any loose pieces 

 of the zinc. Just before making up the cell, dip the zinc into dilute sulphuric acid, 

 wash with distilled water, and dry with a clean cloth or filter paper. 



3. The Zinc Sulphate Solution. — Prepare a saturated solution of pure (" pure 

 recrystallised ") zinc sulphate by mixing in a flask distilled water with nearly 

 twice its weight of crystals of pure zinc sulphate, and adding a little zinc carbonate 

 to neutralise any free acid. The whole of the crystals should be dissolved with the 

 aid of gentle heat, i.e., not exceeding a temperature of 30° C, and the solution 

 filtered, while still warm, into a stock bottle. Crystals should form as it cools. 



4. The Mercurous Sulphate. — Take mercurous sulphate, purchased as pure, and 

 wash it thoroughly with cold distilled water by agitation in a bottle ; drain off the 

 water, and repeat the process at least twice. After the last washing, drain off as 

 much of the water as possible. 



Mix the washed mercurous sulphate with the zinc sulphate solution, adding 

 sufficient crystals of zinc sulphate from the stock bottle to ensure saturation, and a 

 small quantity of pure mercury. Shake these up well together to form a paste of 

 the consistency of cream. Heat the paste sufficiently to dissolve the crystals, but 

 not above a temperature of 30°. Keep the paste for an hour at this temperature, 

 agitating it from time to time, then allow it to cool. Crystals of zinc sulphate 



