A. L. Day and J. K. Clement — Gas Thermometer. 431 



In the determination of the high temperature scale carried 

 out at the Reichsanstalt in 1900, four observations of the 

 expansion of the- bulb material (250°, .500°, 750° and 1000°) 

 were deemed sufficient, and it was not thought necessary in onr 

 earlier observations to increase this number materially. We 

 therefore began with a 200° interval. After the observation 

 at the temperature of the room, the bar was accordingly heated 

 to 200° C. and sufficient time (about 30 minutes) allowed for 

 the temperature to become constant throughout the furnace, 

 after which a temperature reading was made at the middle of 

 the bar with each element. Observations of length were then 

 made in the same order as before upon the pair of lines adjacent 

 to the fixed cross-hair in each of the microscopes, followed by 

 a second temperature reading at the middle of the bar. After 

 these observations of length and before any change was made 

 in the temperature, nine consecutive pairs of observations were 

 made of the temperature distribution along the bar, first at the 

 center, then on the left section at 5, 10 and 12 cm out from 

 the middle, then the center repeated; then upon the right 

 section with similar intervals, and again the center — all with 

 both elements. By this means an accurate measurement of the 

 temperature along the bar corresponding to the length measure- 

 ment just completed was obtained. The whole procedure was 

 then repeated at temperatures of 400, 600, 800 and 1000° C.,* 

 after which the furnace was allowed to cool over night and the 

 length of the bar at the temperature of the room again deter- 

 mined. Immediately following this an observation of the 

 brass bar was made in order to establish the fact that the dis- 

 tance separating the cross-hairs had not been accidentally dis- 

 turbed by the manipulation of the furnace during heating. 



At 800° and 1000° the bar is self-luminous to a sufficient 

 extent to enable measurements to be readily made without out- 

 side light, but it was deemed advisable to use the outside light 

 in the same way at these temperatures also. In passing from 

 outside to inside illumination, the lines are at first dark on a 

 bright ground, and then bright on a dark ground, a change to 

 which the eye accustoms itself only with considerable difficulty. 

 The measurements were therefore much more uniform when 

 outside light was used throughout. 



The measurements of the temperature at once encountered 

 the difficulty that the exposure of the thermoelement in the 

 presence of iridium at a temperature of 1000° contaminates it 

 by an amount sufficient to cause a small but cumulative error. 

 This exposure was necessary with the apparatus as we had 



* Subsequently, when we had reason to suspect an irregularity in the rate 

 of expansion, these observations were repeated every hundred degrees and 

 then every fifty degrees in the region between 600° and 1000°. 



