66 Clark Cell as a Standard of Electromotive Force. [Mar. 17, 



amalgamation of the zinc must go on in the cell to enable it to reach 

 the steady state, and that it may not be sufficient to introduce amalga- 

 mated zincs. On this and some kindred points, however, we are still 

 experimenting. The grey deposit can be shown to be mainly mercury 

 in a state of very fine division. There are some indications that a 

 slight acidity in the solutions is of use in promoting amalgama- 

 tion. 



We have verified repeatedly an observation of Dr. Hopkinson's that 

 the E.M.F. of a bad cell changes considerably if the cell be slightly 

 shaken, while that of a good cell is not affected. 



The paper also contains an account of some experiments on the 

 coefficient of change of E.M.F. with temperature. The value found 

 is 0*000755 per 1° C, practically the same as that given by Lord 

 Rayleigh. In this connexion we may mention the important obser- 

 vation that when the temperature is rising, even although the rise be 

 only a few degrees, the E.M.F. of the cell may — especially if the cell 

 be large — lag very considerably behind the temperature. On one 

 occasion in which the temperature rose by some 5° C. in about a 

 week, the E.M.F. of our large cell at the end of the week corresponded 

 to a temperature nearly 3° lower than that given by a thermometer 

 in the bath with the cell, being about 0*0027 volt too high. In this 

 case a thick cake of crystals had formed on the top of the more solid 

 portion of the paste, and the zinc sulphate solution only attained the 

 state of saturation corresponding to the temperature by very slow 

 degrees. Mr. Carhart and Mr. Swinburne have called attention to 

 the difficulties which thus attend the practical use of the cells. They 

 are to some extent met by using small cells. 



The paper also describes a new form of portable cell which may 

 be turned into any position without harm. Experiments have also 

 been made on the mercury chloride standards described by Von 

 Helmholtz. A set of these has been constructed which has an E.M.F. 

 of very nearly 1 volt. A form of standard due to Grouy, in which 

 oxide of mercury is used, has also been examined. The E.M.F. of 

 these cells prepared with yellow oxide is, we find, 1*381 volts, and 

 when prepared with red oxide 1*388 volts. 



By the kindness of Major Car dew several of our cells have been 

 compared with the standards of the Board of Trade. The differences 

 are very small, being about 0*0003 volt. The average of the Board 

 of Trade cells is less than our standard by about this amount. 



The Board of Trade possess seventy-two cells, and Mr. Rennie, 

 Major Cardew's Assistant, informs us that the greatest difference 

 between any two of them is under 0007 volt. It will be seen from the 

 table given that, while the cells there considered are on the average 

 about one of our units above our standard, they are rather over two 

 of such units above the Board of Trade cells. 



