Nitrogen Thermometer from Zinc to Palladium. 115 



electromotive force used was the true volt, in terms of which 

 the E.M.F. of the Clark cell is 1-4328 at 15°, and of the satu- 

 rated cadmium cell used, 1*01918 volts at 25°. 



Several small corrections are necessary in order to obtain 

 the true E.M.F. of the thermoelement. The calibration cor- 

 rections of the potentiometer (Reichsanstalt calibration) were 

 all negligible except that for the fixed resistance to which the 

 standard cell was attached. This correction amounted to 1*3 

 microvolts in 10,000. The correction for the change of resist- 

 ance with temperature of the potentiometer was also negli- 

 gible. The E.M.F. of the standard cell varies with the 

 temperature ; hence the temperature of the cell was read at 

 each measurement and a small correction applied. The read- 

 ings were correct at 21"5°. For a variation of 5° from this 

 temperature the correction was 2*2 microvolts in 10,000 micro- 

 volts. The resistance of the contacts of the potentiometer, 

 and the small E.M.F.'s existing at contact points in the circuit 

 of the thermoelement, introduced another small error which 

 was determined by placing the thermoelement in ice and read- 

 ing the E.M.F. This correction varied for the different ele- 

 ments from —1 to +4 microvolts. 



As a check upon the absolute value, a Weston standard 

 cadmium cell (calibration by the Bureau of Standards) whose 

 E.M.F. was read directly on the potentiometer, was compared 

 with the saturated cell each day. The agreement of the cor- 

 rected values was usually within 0*5 microvolt. As in the 

 case of the pressure measurement, the absolute value of the 

 E.M.F. is not of importance, since it is used only for trans- 

 ference from the fixed points to the gas thermometer ; the 

 above corrections were applied, however, to reduce the read- 

 ings to a common standard. 



The effect of contamination of the thermoelement wires in 

 furnace readings was much greater than the above mentioned 

 errors.* Up to 1100° the contamination was not serious, but 

 above that temperature the wires take up iridium together with 

 some rhodium. It was hoped that the replacement of iridium 

 in the bulb by rhodium, which is very much less volatile, 

 would do away with this error, but there appeared still to be a 

 very small percentage of iridium in the furnace wire, enough 

 to affect the thermoelement wires appreciably, even though 

 this furnace wire had been especially purified for this purpose. 



Although the task became much longer and more laborious, 

 it was thought wise to make an effort to avoid the error from 

 contamination, even of this diminished magnitude, rather than 

 to attempt to compromise with it by any scheme of approxi- 



*For a more thorough discussion of this effect, see Day and Clement, loc. 

 cit., p. 419 ; and W. P. White, Phys. Eev., xxiii, 449, 1906. 



