CLARK CELL AS A STANDARD OF ELECTROMOTIVE FORCE. 
601 
The Table shows that while Nos. 62 and 64 are about right, Nos. 61 and 63 were 
decidedly too low, though at the time of the latest tests they are all fairly close 
together. Rough tests of the cells had been made on March 20, one or two days 
after they were set up. It will be seen that they were then too low, but that they 
afterwards got worse. 
It thus appears that many of the cells we have examined up to this date began by 
being low, and that this fault increases at first; a grey deposit is also formed on the 
zinc. After a time, however, in most cases a change takes place, and in about a 
month or so the cells have ordinarily come right. In the case of this last set the 
natural development was stopped, and the fault corrected by scraping the zincs and 
removing the deposit. 
It should also be noticed that the solutions are very free from acid. 
§ 20. First Form of Board of Trade Specif cation and Cells Set up in 
accordance with it. 
Early in May a provisional memorandum on the cell was prepared, and on May 25 
six cells, Nos. 65 to 70, were set up by one of us (S.S.) in accordance with it. 
Hopkin and Williams’ mercurous sulphate was used and treated as prescribed in 
the memorandum annexed. 
Memorandum on the Preparation of the Clark’s Standard Cell. 
Definition of the Cell. 
The cell consists of zinc and mercury 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 first be 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 zinc sulphate solution of pure (“pure re-crystallised”) 
zinc sulphate by mixing in a flask distilled water with about twice its weight of crystals of pure zinc 
sulphate, and adding a little zinc car’bonate to neutralise any free acid. The whole of the crystals 
should be dissolved with the aid of gentle heat; and the solution filtered, while still warm, into a stock 
bottle. Crystals will form as it cools, and when the solution is to be used a little should be taken from 
the bottom of the bottle near the crystals with a pipette. While this is being done the liquid should 
be at a decidedly higher temperature than that at which the cells will ordinarily be used. 
4. The Mercurous Sulphate. —Take mercurous sulphate, purchased as pure, and wash it with distilled 
water. Drain off as much of the water as possible. Mix the washed mercurous sulphate in a mortar 
with the zinc sulphate solution, adding sufficient crystals of zinc sulphate from the stock bottle and a 
small quantity of pure mercury. Mix these well together to form a paste of the consistence of cream, 
MDCCCXCII.—A. 4 H 
