THE RADIOMETER. 1 59 



radiant energy, even at that early date. Pringsheim 1 simplified the instru- 

 ment somewhat, suspended the vanes bifilarly with silk thread, and used it 

 to investigate the infra-red spectrum of the sun to about 1.5 /*, produced by 

 means of a glass prism. From this, the first really useful radiometer was 

 developed by Nichols. 2 The behavior of the radiometer has been worked 

 out theoretically by Maxwell 3 in his paper on "Stresses in Rarefied Gases 

 arising from Inequalities of Temperature." Among other things he showed 

 that for two parallel disks very near each other the central points will pro- 

 duce but little effect, because between the disks the temperature varies uni- 

 formly, and only near the edges will there be any stress arising from an 

 inequality of temperature in the gas. It has been shown by others, especially 

 by Crookes and by Nichols, that the sensitiveness of the radiometer is a 

 function of the pressure of the residual gas, of the kind of gas surrounding 

 the vanes, and of the distance of the exposed vanes from the window. The 

 latter, on account of its absorption, of course limits the region of the spec- 

 trum which is to be investigated. If the vanes are not too close to the win- 

 dow, the deflections will be proportional to the energy falling upon one of 



them. 



Comparison of Sensitiveness and Area of Vane. 



For veins of finite dimensions, such as must be used in practical work, 

 the writer has found that the deflections are proportional to the area of 

 the exposed surface of the vane. This is perhaps to be expected, although 

 there seemed to be some doubt. The curve of deflections and exposed 

 area of vanes (area 10.5 X 1.3 mm. for constant pressure of 0.02 mm.) does 

 not pass through the origin. One explanation may be that for infinitely 

 narrow vanes the graph is not a straight line, but curves as it approaches 

 the origin. Because of the impracticability of suspending vanes of different 

 widths successively from the same fiber suspension, at the same distance 

 from the window, using the same gas pressure for all vanes, it was necessary 

 to use one wide vane, with a slit before it, and vary the opening of the slit. 

 The source of energy (Nernst heater) was at a distance of 3 meters and, 

 hence, the width of the projection of the slit upon the vane was practically 

 the width of the slit, except for a very narrow slit when diffraction may 

 decrease the energy incident upon the vane. This, however, would dis- 

 place the graph still farther from the origin. The forces acting in a radi- 

 ometer are so complex and so little understood that no further examination 

 was made to ascertain the limits within which the above proportionality 

 holds. The test was made to reduce the deflections to unit area between 

 the above limits of exposed vane. It is of interest to note in this connec- 

 tion that in a bolometer the sensitiveness varies as the square root of the 

 area of the bolometer strip. 



1 Pringsheim, Ann. der Phys. (3), 18, p. 32, 1883. 



2 Nichols: Phys. Rev., 4, p. 297, 1897; Berl. Ber., p. 1183, 1896. 



3 Maxwell: Collected Papers, 2, p. 681; Phil. Trans., Part I, 1879. 



