A. L. Day and J. K. Clement — Gas Thermometer. 42T 



the side openings, narrow slits about 3 mm in width by 10 mm long. 

 The tube and its heating coil extended some 10 cm beyond the 

 ends of the bar and the wire was wound somewhat more 

 closely at the ends than in the middle to counteract the cooling 

 effect of the end and side openings. In this way a reasonably 

 uniform distribution of temperature along the bar was secured. 

 The first furnace tube was of porcelain wound with nickel 

 wire l mm in diameter, the separate turns being insulated from 



Fig. 5. . 



Fig. 5. A section through the furnace (A) at one of the openings showing 

 method of illumination by 45° plane glass plate (a). The bar and thermo- 

 element appear in position though not well shown by this section. 



each other with a magnesite cement which is sufficiently 

 refractory and conducts but little at any temperature which 

 the nickel wire can withstand. Thus arranged, the heating 

 coil was mounted horizontally in a much larger tube (8 cm diam- 

 eter) of porcelain and the space between filled with dry 

 calcined magnesia of good insulating quality. The whole was 

 water-jacketed throughout in order to prevent any heat 

 from the furnace from entering the optical system and disturb- 

 ing the fixed distance between the micrometers upon which 

 the accuracy of the measurement absolutely depends. Both 

 the insulating material and the water jacket were provided with 

 small openings corresponding to the slits in the furnace tube 

 so that the bar could be illuminated and observed from without. 

 The measuring portion of the apparatus was entirely separate 

 from the furnace and consisted. of two telescopes, mounted upon 



