Wheatstone bridges and accessory apparatus for resistance thermometry

Abstract

This chapter provides an overview of blackbody radiation. All objects that have a temperature at any value other than absolute zero continuously emit and absorb radiation. The radiation characteristics of certain surfaces are completely specified if the temperature is known. These surfaces radiate continuously through the optical spectrum and are known as ideal thermal radiators or blackbodies. Blackbody radiation is described by Planck's equation. Slide rules are available that provide rapid calculation of blackbody quantities with good accuracy. The Planck radiation formula shows that the spectrum of the radiation shifts toward shorter wavelengths as the temperature of the radiator is increased. The derivative of the Planck equation with respect to wavelength yields the Wien displacement law that gives the wavelength for which maximum radiation occurs for a given temperature. The total power radiated per unit area of a blackbody is obtained by integrating the Planck's radiation law over all wavelengths. Two well-known approximations to Planck's law are readily obtained: Rayleigh-Jeans' law and Wien's radiation law. The chapter also highlights emissivity, Kirchhoff's law, and Lambert's cosine law. Emissivity of a surface is a function of wavelength, temperature, and direction. Lambert's cosine law signifies that the amount of energy in a given solid angle varies in proportion to the cosine of the angle between the direction in question and the normal to the surface.