Nitrogen Thermometer from Zinc to Palladium. 107 



measurable difference found. Similarly, repacking the bulb 

 in a fresh supply of ice gave exactly the same value. 



B. Definition of Temperature by Measurements of Pres- 

 sure. — The procedure in measuring the pressure, p', was as 

 follows : First the three mercury thermometers on the mano- 

 meter were read to determine the temperature of the mercury 

 column and scale ; then three to four settings of the barometer 

 were made, alternating with measurements of the manometer. 

 The mercury thermometers were read again at the close. Dur- 

 ing this interval the other observer made as many readings as 

 possible of all the thermoelements. 



Before the manometer was connected to the bulb, the point 

 on the scale corresponding to the reference point of the mano- 

 meter* was determined once for all before the manometer was 

 connected to the bulb, by connecting the two arms and raising 

 the mercury to the point, as in a regular pressure measure- 

 ment. Subsequent manometer readings were subtracted from 

 this fixed level, and the resulting difference corrected for the 

 temperature and calibration corrections of the scale and then 

 reduced to 0°. The barometer reading was similarly corrected. 

 The algebraic sum of the two gave the pressure^/, in terms of 

 a centimeter of mercury at 0° and at the latitude and elevation 

 of the laboratory. Since the absolute value of the pressure 

 does not enter into the gas thermometer formula, corrections 

 for altitude and latitude are superfluous. 



.Errors and Corrections in ft '. — The level of the fixed refer- 

 ence point of the manometer varies with the temperature of 

 the room because of the difference in expansion of the brass 

 scale on the one hand and of the glass tube of the manometer 

 which carries the fixed point on the other. This correction 

 can be calculated from the expansion coefficients of the mate- 

 rials and amounts to 0*04 mm per 5°. Its direction and amount 

 were checked experi men tally by determining the fixed point at 

 two temperatures differing by about 10°, the room being open 

 on a cold day for the one case, and then closed and heated for 

 the other. The difference found was 0*09 mm , and that calcu- 

 lated 0-08 mm . 



The lengths of the divisions of the brass scale were corrected 

 for change of temperature by a formula determined for this 

 scale at the N ormal-Aichuugs-Kommission, the absolute length 

 of the scale having been determined at 16°. In addition, cali- 

 bration coiTections, determined for each millimeter of the scale, 

 were applied. The total scale correction was always less than 

 0*15 mm , hence the temperature measurement by the adjacent 

 mercury thermometers was abundantly accurate for this pur- 

 pose. 



*p. 100. 



