THE PNEUMATIC PYROMETER. 1 53 



continuously for temperatures above 2500, but at that tem- 

 perature and lower it works well and satisfactorily for months 

 without requiring any readjustment. The automatic register 

 is very sensitive, and can be easily adjusted for a new range of 

 temperatures at any time. 



An explanation of the principle of its working is given in 

 the inventor's own words: 



1 ' The Pneumatic Pyrometer is based on the laws govern- 

 ing the flow of air through small apertures. 



"If two such apertures A and B (Fig. 38) respectively 

 form the inlet and outlet openings of a chamber C, and a uni- 

 form suction is created in the chamber C' by the aspirator D, 

 the action will be as follows : 



4 'Air will be drawn through the aperture B into the 

 chamber C', creating suction in chamber C, which in turn 

 causes air from the atmosphere to flow in through the aper- 

 ture A. The velocity with which the air enters through A 

 depends on the suction in the chamber 7, and the velocity 

 at which it flows out through B depends upon the excess of 

 suction in C' over that existing in the chamber C, that is, the 

 effective suction in C' . As the suction in C increases, the 

 effective suction must decrease, and hence the velocity at 

 which air flows in through the aperture A increases, and the 

 velocity at which air flows out through the aperture B de- 

 creases, until the same quantity of air enters at A as passes 

 out at B. As soon as this occurs no further change of suc- 

 tion can take place in the chamber C. 



"Air is very materially expanded by heat. Therefore 

 the higher the temperature of the air the greater the volume, 

 and the smaller will be the quantity of air drawn through a 

 given aperture by the same suction. Now if the air as it 

 passes through the aperture A is heated, but again cooled to 

 a lower fixed temperature before it passes through the aper- 

 ture B, less air will enter through the aperture A than is 

 drawn out through the aperture B. Hence the suction in C 



