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



observe these divisions and also to maintain a constant distance 

 between the fixed cross-hairs from beginning to end of the 

 experiment. Heating the bar then served to move the ruled 

 lines past the fixed cross-hairs of the telescopes and the amount 

 of the. displacement was measured for any desired temperature. 



Fig. 4. 



Fig. 4. Section through furnace showing bar, thermoelements (E,E) and 

 microscopes in position. A section through the arrow is shown in fig. 5. 



The aggregate expansion of a 25 cm bar over the interval from 

 0° to 1000 b is about 2*5 mm . The telescope micrometers as. they 

 were focused for the measurements gave about 450 divisions 

 (each about 2 mm ) of the drum for one millimeter on the bar, 

 and in the individual readings differences of "2 or *3 of a divi- 

 sion were readily distinguishable. It was therefore easily 

 possible to make very accurate measurements of the expansion 

 of such a bar by direct observation without the use of a con- 

 tact lever or any multiplying device whatsoever. 



The essential features" of the apparatus can be partly seen 

 from the figures and partly require some description. The 

 furnace was erected on a separate stand quite independent of 

 the measuring apparatus. It consisted of a narrow tube 

 wound with a heating coil and containing, opposite the ends of 

 the bar, two small openings through which the divisions could 

 be seen. The inside diameter of the tube was about 2 cm , and 



