1892.] The Clark Cell as a Standard of Electromotive Force. 63 



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 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 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 0*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. 



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 pre- 

 pared 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. 



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. 



These cells, as the tests given show, have been good from the first, 

 and, indeed, we have not had any difficulty with any of the cells in 

 which the instructions of this memorandum have been followed. 



The mercury used had been distilled in the laboratory, the zincs 

 were supplied as " pure " by Messrs. Harringtons, of Cork, while 

 the zinc and mercurous sulphates came from Messrs. Hopkin and 

 Williams. 



The numbers in the table show the differences between the cells 

 and the standard ; the unit is '00025 volt. 



