When rhodium is heated to incandescence in an atmosphere of air or oxygen, a coating forms on its surface. This is shown by a change in the emissivity. When the temperature is increased sufficiently this coating suddenly disappears, and when the temperature is lowered it reappears. This coating is probably an activated adsorption of oxygen, or an oxide of rhodium. It is found experimentally that the temperature and partial oxygen pressure at which the coating disappears are related by the Clausiushyphen;Clapyron equation:lpar;dthinsp;logePrpar;sol;dTequals;Lsol;RT2.P is the partial oxygen pressure,Tthe absolute temperature, andLthe heat of vaporization of the surface. The electron work function of a clean rhodium surface is found to beopen phiequals;4.80volts, andA= 33 amp./cm2/degree2.
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