ON STANDARDS FOR USE IN ELECTRICAL MEASUREMENTS. 141 



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 containing 

 the platinum wire should project some way above the top of the marine 

 glue. 



The cell may be sealed in a more permanent manner by coating the 

 marine glue, when it is set, with a solution of sodium silicate, and leaving 

 it to harden. 



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. 



The form of the vessel containing the cell may be varied. In the 

 H form the zinc is replaced by an amalgam of ten parts by weight of zinc 

 to ninety of mercury. The other materials should be prepared as already 

 described. Contact is made with the amalgam in one leg of the cell and 

 with the mercury in the other by means of platinum wires sealed through 

 the glass. 



Notes to the Specification on the Preparation of the 



Clark Cell. 



The Mercurous Sulphate. — The treatment of the mercurous sulphate 

 has for its object the removal of any mercuric sulphate which is often 

 present as an impurity. 



Mercuric sulphate decomposes in the presence of water into an acid 

 and a basic sulphate. The latter is a yellow substance — turpeth mineral 

 — practically insoluble in water ; its presence at any rate in moderate 

 quantities has no effect on the cell. If, however, it is formed, the acid 

 sulphate is formed also. This is soluble in water, and the acid produced 

 affects the electro-motive force. The object of the washings is to dissolve 

 and remove this acid sulphate, and for this purpose the three washings 

 described in the specification will in nearly all cases suffice. If, however, 

 a. great deal of the turpeth mineral is formed, it shows that there is a 

 great deal of the acid sulphate present, and it will then be wiser to obtain 

 a fresh sample of mercurous sulphate rather than to try by repeated 

 washings to get rid of all the acid. 



The free mercury helps in the process of removing the acid, for the 

 acid mei'curic sulphate attacks it, forming mercurous sulphate and acid 

 which is washed away. 



Pure mercurous sulphate when quite free from acid shows on repeated 

 washing a faint primrose tinge, which is due to the formation of a basic 

 mercurous salt, and is distinct from the turpeth mineral or basic mercuric 

 sulphate. The appearance of this primrose tint may be taken as an 

 indication of the fact that all the acid has been removed, and the washing 

 may with advantage be continued until this primrose tint appears. Should 

 large quantities of this basic mercurous salt be formed, the sulphate should 

 be treated as described in the instructions for setting up Clark's cells 



