CLARK CELL AS A STANDARD OF ELECTROMOTIVE FORCE. 
627 
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 ci'ystals of pure zinc 
sulphate, and adding about 2 per cent, of zinc oxide to neutralize any free acid.* The crystals should 
be dissolved with the aid of gentle heat, but the temperature to which the solution is raised should not 
exceed 30° C.; about 12 per cent, of mercurous sulphate treated as described in 4 should be added, 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 consistence of cream. Heat the paste, but not above a tempera¬ 
ture of 30°. Keep the paste for an hour at this temperature, agitating it from time to time, then allow 
it to cool; continue to shake it occasionally while it is cooling. Crystals of zinc sulphate should then 
be distinctly visible, and should be distributed throughout the mass ; if this is not the case, add more 
crystals from the stock bottle, and repeat the whole process. 
This method ensures the formation of a saturated solution of zinc and mercurous sulphates in water. 
Contact is made with the mercury by means of a platinum wire about No. 22 gauge. This is pro¬ 
tected 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 poi’tion 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 centimetres diameter, and 6 or 
7 centimetres deep. Place the mercury in the bottom of this tube, filling it to a depth of, say, P5 centi¬ 
metre. Cut a cork about - 5 centimetre 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 centimetre 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. 
Shake up the paste and introduce it without contact with the upper part of the walls of the test tube, 
filling the tube above the mercury to a depth of rather more than 2 centimetres. 
Then insert the cork and zinc rod, passing the glass tube through the hole prepared for it. Push the 
cork gently down until its lower surface is nearly in contact with the liquid. The air will thus be 
nearly all expelled, and the cell should be left in this condition for at least 24 hours before sealing, 
which should be done as follows :— 
Melt some marine glue until it is fluid enough to pour by its own weight, and pour it into the test 
tube above the cork, using sufficient to cover completely the zinc and soldering. The glass tube should 
project above the top of the marine glue. 
The cell thus set up may be mounted in any desirable manner. It is convenient to arrange the 
mounting so that the cell may be immersed in a water bath up to the level of, say, the upper surface of 
the cork. Its temperature can then be determined more accurately than is possible when the cell is 
in air. 
In using the cell sudden variations of temperature should as far as possible be avoided. 
* See notes at the end of the memorandum. 
4 L 2 
