Improved Radiometer Construction

Construction of more sensitive Nichols radiometer.—The sensitivity of the Nichols radiometer was increased by a factor of five by attaching "pearled" mica shields immediately behind the vanes. Optimum spacing of vane and shield was experimentally determined. Further increase in sensitivity was obtained by increased lightness of construction and by choice of dimensions and working conditions. Deflections were found proportional to energy, and the sensitivity per unit area proved independent of vane width for vanes less than 1.5×.5 mm. The most efficient fibre was found to be one which gives a half-period equal to the desired deflection time, and the best gas pressure that which gives critical damping. The sensitivity obtained is 2.1 ${\mathrm{}\left(10\right)\mathrm{}}^{-8\mathrm{}}$ watts/${\mathrm{cm}}^2$ for 1mm deflection. Thin platinum films make good absorbing surfaces for radiometer vanes. Absorption of long infra-red waves by thin platinum films was found to reach a sharp and high maximum for a definite thickness, which, however, varies with the wave-length.