A. L. Day and J. K. Clement — Gas Thermometer. 443 



the equation may be transformed into a more convenient form 

 for computations : 



p 1 + 30* 



+ at = 



^1+A-*B 



1\ 

 Here 3/3t represents the correction for the expansion of the 



bulb and A — — B is the correction for the unheated space. 



In computing p the mercury columns were corrected in the 

 usual manner for temperature and latitude. 



Gas Thermometer Measurements. — Table I contains some 

 of the earlier results, which were obtained after the temper- 

 ature gradient along the bulb had been partially corrected. 

 During this series of observations, the temperature difference 

 between the middle and either end of the bulb varied between 

 50 and 150 M.V. (5°- to 15°-). As it was impossible entirely to 

 eliminate the gradient with the arrangement of coils in use at 

 this time, the heating currents were adjusted so as to have the 

 gradients toward the top and bottom of the bulb of opposite 

 sign and of nearly equal value, thereby materially reducing 

 the magnitude of the correction to be applied. 



Before beginning this series of observations and again after 

 its completion, the thermo-couple "P" was calibrated by deter- 

 mining its electromotive force at the zinc and copper melting 

 points. From the results which follow it will be seen that the 

 electromotive force of the thermoelement, at the temperature 

 of melting copper, has been lowered 15 M.Y. (1*2°) through 

 iridium contamination during the series of measurements. 



1906 Zinc. Copper. 



April 28 F.* 3398 10488 



M. 3398 10483 



3398 10486 



May 14 F. 3396 10469 



M. 3398. 10472 



3397 10471 



After these observations (Table I) the furnace was rebuilt. 

 In place of the heating coil of platinum -iridium alloy, a coil 

 of pure platinum was substituted. At the same time the 

 arrangement of the two auxiliary heating coils was so modi- 

 fied that by proper adjustment of rheostats, the gradient along 

 the bulb could be reduced to 0*5° or less. 



In order to eliminate, as far as possible, any error due to the 

 contamination of the thermo-couples with iridium, the couples 



* F = Freezing point, M = Melting point. 



