140 



Prof. H. L. Callenclar and Mr. H. T. Barnes. 



We also found it possible to obtain observations of tlie change of 

 E.M.F. of cells saturated at 15° and 30° C, under conditions in which 

 the solution was considerably supersaturated. The cell saturated at 

 15° C. was kept for several hours at 0° C. without showing any trace 

 of crystallisation. This cell agreed at 15° C. with the saturated cells, 

 and gave very consistent readings throughout the range 0° to 30° C. 

 The change of E.M.F. per 1° C. was found to be 0*567 of a millivolt 

 between 0° and 15° C., and 0*560 of a millivolt between 15° and 30°. 

 As the whole change of E.M.F. between 0° and 30° C. was only 16 90- 

 millivolts, and as the E.M.F. of the cell rose by one- tenth of a milli- 

 volt after keeping for six hours at 0° C, the observations may be 

 taken as showing that the temperature- coefficient of this cell, whether 

 in the supersaturated or unsaturated condition, is practically constant 

 over the range 0° to 30° C. This is in marked contrast with the case 

 of the cells containing crystals. 



The tests of the cells saturated at 0° C. were very fairly consistent,, 

 but not quite so good as those of the cells saturated at 15° C. They 

 showed a mean temperature change of E.M.F. per 1° C. of 0*543 of a 

 millivolt. There was no decided evidence of any variation of the 

 temperature-coefficient over the range 0° to 30° C. Car-hart gives 

 the formula : — 



E, = E 15 [1 - 1-000387 (£-15) +0*0000005 (*-15) 2 ], 



which would make the temperature-coefficient diminish slightly as 

 the temperature rises. (Change of E.M.F. 0*56 mv. per 1° C). 



The tests on the weaker cells were much less consistent, owing to 

 the instability of E.M.F. above referred to. The results of the tests 

 pointed to a mean change of 0*55 of a millivolt per 1° C. The change 

 observed between 0° and 15° C. was sometimes greater and sometimes 

 less than that between 15° and 30° C, but there was no decided ten- 

 dency either way. After keeping for some time at 0° or 30° C. these 

 weak cells sometimes showed permanent changes amounting to as- 

 much as half a millivolt. 



§ 21. On the Density of Solutions of Zinc Sulphate. 



A knowledge of the density of solutions of zinc sulphate is required 

 in order to trace the relation between the changes of E.M.F., which 

 depend on change of strength and density of the solution. This 

 point has been investigated by two of Professor Carhart's students,, 

 the result of whose work has been published by Professor Carhart.* 

 These observers find for a cell saturated at 15° C. an E.M.F. nearly 

 five millivolts higher than that of a cell containing crystals, and a 

 density which appears to be correspondingly low. For this and 



* ' Proc. Amcr. Elect. Eng..' 1802. p. 615. 



