584 
MESSRS. R„ T. GLAZEBROOK AND S. SKINNER ON THE 
their latest form, June 23, 1891, are given here. Fig. 2 shows the arrangements of 
the parts of the cell as described in the memorandum. 
Memorandum on the Preparation or 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 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 re-crystallised ”) 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 shonld fonn 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 consistence 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 should then be distinctly visible 
throughout the mass; if this is not the case, add more crystals from the stock bottle, and repeat the 
process. 
This method ensures the formation of a saturated solution of zinc and mercurous sulphates in water. 
The presence of the free mercury throughout the paste preserves the basicity of the salt, and is of the 
utmost importance. 
Contact is made with the mercury by means of a platinum wii’e about No. 22 gauge. This is 
protected from contact with the other materials of the cell by being sealed into a glass tube. The ends 
of the wire project from the ends of the tube ; one end forms the terminal, the other end and a portion 
of the glass tube dip into the mercury. 
To set up the Cell. 
The cell may conveniently be set up in a small test tube of about 2 cm. diameter, and 6 or 7 cm. 
deep. Place the mercury in the bottom of this tube, filling it to a depth of, say, l - 5 cm. Cut a cork 
about '5 cm. thick to fit the tube; at one side of the cork bore a hole through which the zinc rod can 
pass tightly; at the other side bore another hole for the glass tube which covers the platinum wire; at 
the edge of the cork cut a nick through which the air can pass when the cork is pushed into the tube. 
Pass the zinc rod about 1 cm. through the cork. 
Clean the glass tube and platinum wire carefully, then heat the exposed end of the platinum red hot 
and insert it in the mercury in the test tube, taking care that the whole of the exposed platinum is 
covered. 
