LONDON Physical Society, March 2. A. O. RANKINE: A simple method of demonstrating the paramagnetism and diamagnetism of substances in magnetic fields of low intensity (see NATURE, 133, 150, Jan. 27,1934). A. M. FERASAH: Anomalous changes in temperature due to thermionic emission in the filaments of valves. In some valves the steady filament temperature is lower when the anode is positive, as would be expected, but in other valves it is higher. This anomalous increase in temperature is due to radiation from the anode and is larger for valves which have a high anode dissipation and an anode which closely surrounds the filament. After correction for this effect has been applied, the work-function can be approximately calculated from measurements made on an ordinary valve. T. SMITH: Change of variables in Laplace's and other second-order differential equations. Transformations of variables are expressed as matrix products, the effect of transposition being particularly considered, and the results are applied to the transformation of the general second-order differential expression. MABY TAYLOR: The Apple-ton-Hartree formula and dispersion curves for the propagation of electromagnetic waves through an ionised medium in the presence of an external magnetic field. (2) Curves with collisional friction. Four typical frequencies have been chosen for the calculations, one from each of the classes into which the frequencies fall when collisional friction is absent, as described in part 1. The corresponding wavelengths are 80, 240, 400 and 1,000 metres. The various stages in the effect of increasing collisional friction have been found to be usefully represented by collisional frequencies of 105, 106, 107 c./sec. and curves are given showing the indices of refraction KT (r=a,b), and the real part and imaginary part of Mr2 or (yLr-iKrc/p)2, together with the polarisations of the basic modes as functions of the electronic density for each of the four frequencies and collision frequencies named. The process of evaluation of Mr and of the polarisation is described. The attenuation and absorption are found to be, in general, greater for the right-handed component than for the left-handed component, with the direction of magnetic field appropriate for down-coming waves in the northern hemisphere. The use of the dispersion curves in the interpretation of propagation phenomena is discussed. J. McGARVA BRTJCKSHAW: An instrument for electrical prospecting by the inductive method. In the Bieler-Watson method of geophysical surveying, in general, the horizontal field is not in quadrature with the vertical field. An instrument has been designed which will allow the horizontal field to be compared completely with the vertical field, an important feature being that the horizontal components in phase and in quadrature with the vertical field are obtained directly from the instrument readings. The apparatus has been tested on elliptically polarised fields and has given satisfactory results.