H. & Carhart— One-Volt Standard Cell -61 



It is now well known that the E. M. F. of a voltaic cell with 

 zinc immersed in a solution of one of its salts decreases with 

 increase of density of the solution.* Within limits therefore 

 the E. M. F. of a cell can be varied by changing the density of 

 the solution of the zinc salt. Thus the E. M. F. of a Clark 

 cell was found to vary from 1-434 volts with a solution saturated 

 at 20° C. to 1*481 volts with a 5 per cent solution, a variation of 

 nearly three and one-third per cent. 



The variation of the E. M. F. with density is even greater 

 in the case of zinc chloride. With zinc chloride 16 per cent 

 increase of density produces 3 5 per cent decrease in E. M. F. 

 An equal increase of density of zinc sulphate is accompanied 

 by only half the percentage decrease of E. M. F. Hence 

 greater care is necessary in standardizing a solution of zinc 

 chloride than of zinc sulphate. 



Taking the Clark cell with excess of crystals of zinc sulphate 

 as the standard, having an E. M. F. of 1*434 volts,*)* at 15° C, 

 I have found that the density of the zinc chloride solution 

 required to give one volt is 1*391 at 15° C. Ostwald's density 

 gives too low a value. It was probably adjusted to correspond 

 with the legal ohm. 



The cell is made in the same form as my Clark standard. 

 In the bottom of the tube is pure mercury in contact with a 

 platinum wire; on this a paste of mercurous chloride and the 

 zinc chloride solution ; a cork diaphragm follows, holding the 

 mercury and paste firmly in position, especially with some 

 asbestos packing under the cork ; zinc chloride is then added 

 to the proper depth, and an amalgamated zinc rod, supported 

 by a cork, completes the electrical combination of parts. The 

 cell must be hermetically sealed as usual. Such a cell is per- 

 fectly portable and gives promise of long life. Its internal 

 resistance is about 15U0 ohms, and it does not appear to suffer 

 permanent change by heating to 50° C. or even to 60°. The 

 cell six years old already mentioned is still in good condition 

 and has apparently maintained its E. M. F. 



An interesting feature of this cell is its small positive tem- 

 perature coefficient. It is well known that the coefficient of 

 the Clark and the Daniell standard is negative. The calomel 

 cell has a coefficient of about + -00009 within working limits 

 of temperature. In the following table will be found the data 

 of one series of observations on the temperature coefficient : 



* Carhart, this Journal. Nov. 1884, p. 314. 



f Report of the B. A. Committee, Edinburgh Meeting. 1892. 



