Evolution of II cat by Pitchblende. 317 



means of a cell of known voltage, using a standard 0*02 ohm 

 manganin resistance, and measuring the E.M.F. of the couple 

 when there was a known difference of temperature (a few 

 degrees) between the junctions. 



Hence, knowing the resistances of the galvanometer and 

 the couple, w r e at once obtain the fraction of a degree which 

 corresponds to one division of the galvanometer-scale. It 

 was found that the galvanometer was deflected (on reversal) 

 13*32 scale-divisions per microvolt applied to its terminals, 

 the arrangement being such that thermo-electric effects were 

 as far as possible eliminated. The E.M.F. of the couple used 

 in the pitchblende experiments was found to be 159*7 

 microvolts per degree. This is the open circuit E.M.F. as 

 the arrangement adopted was a nul method such that when 

 the E.M.F. was measured no current was being taken from 

 the couple. The resistance of the galvanometer being 

 3O0 ohms and that of the couple 25'4 ohms, 1° C. corre- 



. , 159- 7 x 13*32 x30-0 lt - g , ,. . . 



sponds to r^— ; =11:)2 scale-divisions. 



1 05*4 



The other method consists in placing one terminal of the 

 couple, encased in a glass tube, in a vessel of planed ice and 

 water which can be subjected to varying pressures by means 

 of a mercury reservoir and a flexible tube. The reservoir 

 can be raised or lowered by a cord passing over a pulley and 

 the level of the mercury in it read on a vertical scale. As 

 the pressure vessel is completely filled with ice and water. 

 the variation of the level of the mercury in it is negligible. 

 The vessel is buried in ice in tin; zinc ice-vessel. The stem 

 of the couple passes through a rubber stopper which is 

 firmly tied down and the outer junctions are buried in 

 the ice. 



A reading of the galvanometer deflexion having been 

 made with the reservoir at a certain height, the latter is 

 raised a known distance and the variation in the galvano- 

 meter deflexion noted. For every 76 cm. that the reservoir 

 is raised the temperature of the ice and water is lowered 

 0°'0074b' C; and hence by noting the change of the gaivano- 

 nometer deflexion the sensitivity of the apparatus is found, 

 since the temperature of the outer junction remains constant. 

 After taking a reading with the reservoir raised, the latter is 

 again lowered to the standard position. The galvanometer 

 reading generally did not return exactly to its original value. 

 Accordingly the mean of the galvanometer readings, with 

 mercury at standard height, obtained immediately before and 

 after each reading with mercury raised, is subtracted from the 

 latter, and the ditference entered as the deflexion produced 



