Table of contents

Experiments are discussed establishing the relation of the absolute thermodynamic scale of temperature below 1° k. to the magnetic scale obtained by the extrapolation of Curie's law. The experimental results on iron ammonium alum, manganous ammonium sulphate, and chrome potassium alum are compared with the theoretical calculations of Onsager and Van Vleck.

A general theory of the reflectivity and emissivity of materials is derived in a conveniently tractable form. As an example of the power of the method, it is applied to the Drude-Zener treatment of the optical properties of an ideal metal containing perfectly free electrons only. On this basis it is shown that there should exist a wide region of constant reflectivity and emissivity in the near infra-red. Krönig's theory is shown to be an approximation to the exact expression. The temperature coefficient of reflectivity is investigated and it is seen, on certain general assumptions, that it cannot change sign except in the neighbourhood of a wavelength at which the metal becomes transparent.

Disagreement with experimental observations indicates that the Drude-Zener treatment must be modified, either by a fundamental change in the dispersion formula or, more probably, by assuming that the influence of the bound electron contribution to the optical behaviour of a metal is greater than had been supposed.

Measurements of the electrical conductivity of germanium are described. The results can be explained by the theoretical calculations of Shifrin (1944). An account is given of the relevant part of Shifrin's work. The experimental results are used to deduce the concentration of impurity centres and of thermally excited electrons and the position of the impurity levels.

The half-life of ⁶He is found to be 0.823±0.013 sec. by a rotating wheel method. The statistical accuracy is considerably higher than that of previous measurements, though all results agree to within their limits of error.

The disintegration of ¹²C into three α-particles by inelastic scattering of fast neutrons has been studied by the photographic plate method. Measurements made on 168 of the stars formed by this disintegration have been used to identify the reaction and to obtain values of the cross-section for the process at neutron energies between 10.8 MeV. and 14.5 MeV. No evidence was found for anisotropic scattering of the neutrons.

This direct method of measuring nuclear spin-lattice relaxation times is based on Bloch's nuclear induction experiment in which the material investigated is subjected to a magnetic field that is varied several times per second through the value for nuclear resonance. Under certain conditions a simple calculation may be made of the relation between the magnitude of the nuclear induction effect, the relaxation time, and the time intervals between successive passages through resonance. Experimental observation of this relation then determines the relaxation time directly.

An expression is derived for the displacements in an isotropic elastic medium which contains an edge dislocation moving with uniform velocity c. When c=0 the solution reduces to that given by Burgers for a stationary edge dislocation. The energy density in the medium becomes infinite as c approaches c₂, the velocity of shear waves in the medium; this velocity therefore sets a limit beyond which the dislocation cannot be accelerated by applied stresses. The atomic structure of the medium is next partly taken into account, following the method already used by Peierls and Nabarro for the stationary dislocation. The solution found in this way differs from the one in which the atomic structure is neglected only within a region of width ζ which extends not more than a few atomic distances from the centre. ζ varies with c and vanishes when c=c_r, the velocity of Rayleigh waves. It becomes negative when c_r< c<c₂. Thus c_r rather than c₂ appears to be the limiting velocity when the atomic nature of the medium is taken into account. Since c_r similar, equals 0.9c₂ the difference is not of much importance.

The same method applied to a screw dislocation gives, in the purely elastic case, the expression already derived by Frank. The corresponding Peierls-Nabarro calculation shows that the width ζ is proportional to (1 - c²/c₂²)^½. This "relativistic" behaviour is analogous to Frenkel and Kontorowa's results for their one-dimensional dislocation model.