Electromagnetic theory and the experiments of Hagen and Rubens lead to the formula $\frac{\delta E}{E}=\frac{\delta \sigma }{\left(2\mathrm{}\sigma \right)}$, where $\delta E$ is the change produced in the emissivity $E$ of a metal for long heat waves by a change $\delta \sigma$ in the electrical conductivity $\sigma$. Using a thermopile to detect $\delta E$ a magnetic field of 4900 gauss was used to produce the $\delta \sigma$ in bismuth plates kept at about 100°C. No effect of the field on $E$ was observed for polished surfaces, surfaces etched with nitric acid, or surfaces of a plate cast in vacuum. A change of $E$ ten times smaller than the expected result could have been detected. Possible causes of the negative result are (1) the presence of emitted energy of too short a wave-length, (2) the absence of magneto-resistance in bismuth for high frequency currents, and (3) the absence of magnetoresistance in surface layers of bismuth. Reasons are given for rejecting (1) and (2) and accepting (3) as an explanation of the experiment.

• Received 20 March 1926

DOI:https://doi.org/10.1103/PhysRev.27.764