ON STANDARDS FOR USE IN ELECTRICAL MEASUREMENTS. 133 



Preparation of the Materials. 



1. Tlie 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 redistilled 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 Sidphate 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 zinc oxide in the proportion of about 2 per cent, by weight of 

 the zinc sulphate crystals to neutralise 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. Mercurous 

 sulphate treated as described in 4 should be added in the proportion of 

 about 12 per cent, by weight of the zinc sulphate crystals, and the solu- 

 tion filtered, while still warm, into a stock bottle. Crystals should form 

 as it cools. 



4. Th& 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 temperature of 30° C. 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 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 mercui-y. 



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, 1'5 centimetre. 

 Cut a cork about '6 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 



' See Notes. 



