ON STANDARDS FOR USE IN ELECTBICAL MEASUREMENTS. 159 



a little zinc carbonate to neatralise any free acid. The whole of the 

 crystals should bo 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 Mercuro'U'i Sulphate. — Take mercurous sulphate, purchased as 

 pure, and wash it thoi'oughly with cold distilled water by agitation in a 

 bottle ; drain olF 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 Ijottle, and repeat the jarocess. 



This method insures 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 imjaortance. 



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 euds 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, 1'5 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 wii^e carefully, then heat the 

 exposed end of the platinum red hot, and insert it in the mercurv 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 cm. 



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 twenty- 

 four 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. 



