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



junctions could be .bound together and moved freely along the 

 bar and in contact with it, in order to give a double reading of 

 the temperature at any point desired. In this way we obtained 

 the actual distribution of temperature along the bar corre- 

 sponding to each determination of its length. 



To complete the system, a standard brass bar was prepared 

 of the same size and shape as the platin-iridium bar under 

 investigation, but with silver surfaces let in at the ends to 

 carry the divisions. This bar was compared at 20° C. with 

 the standards of length at the Bureau of Standards, and 

 served to establish the absolute distance separating the cross- 

 hairs before and after each set of observations. 



The method of procedure was now substantially as follows : 

 The standard brass bar was placed in position in the furnace at 

 the temperature of the room. All the necessary adjustments 

 to secure good illumination, to bring the cross-hairs parallel to 

 the scale divisions, and to bring the lines into sharp focus, 

 were then made once for all, and these adjustments were never 

 again disturbed until the series was completed. The field of 

 the microscopes included 5 mm of the bar, but only the three 

 scale divisions bounding the 2 mm nearest to the fixed cross-hair 

 were used. Toward the close of the investigation, for an 

 important reason which will presently appear, only the two 

 bounding divisions of the single millimeter which included the 

 fixed cross-hair were read and all the observations which had 

 been made outside this limited region were rejected. Readings 

 were made from left to right in each microscope and then 

 repeated in the reverse direction to obviate errors from the 

 micrometer screw. The temperature for this measurement 

 was determined with a glass thermometer thrust into one end 

 of the furnace tube adjacent to the bar and read before and 

 after the series of micrometer readings. This observation 

 served to establish in absolute measure the distance apart of 

 the fixed cross-hairs of the microscopes. The brass bar was 

 then removed and the platin-iridium bar corresponding to the 

 gas thermometer bulb inserted in its place in the same relative 

 position. It is necessary here again to emphasize the fact that 

 all further adjustment must be made with the bar and not with 

 the optical parts of the apparatus. 



Having brought the bar into exactly the same position with 

 respect to the telescopes which the brass bar previously occur 

 pied, and having introduced the thermoelements in such a way 

 that their hot junctions were free to travel along the bar from 

 end to end without disturbing it, a second series of observa- 

 tions at the temperature of the room was made in the same 

 way as before. This yields the absolute length of the bar at 

 room temperature in terms of the standard brass bar. The 

 furnace is then ready for heating to the temperatures desired. 



