Table of contents

The propagation of ultrasonic energy in a nickel single crystal by an improved pulse technique has been employed to investigate the behaviour of the dynamic elastic constants of, and the energy losses in the specimen, with and without an applied magnetic field. The dynamic elastic constants behave according to Zener's theory of anelasticity in the test range of frequencies. Values of the relaxation time for internal atomic rearrangement and ratios of the relaxation strengths for shear and compressional waves are evaluated for the sample. Energy losses are substantially reduced when the sample is magnetized to saturation.

A macroscopic phenomenological relationship is proposed, which takes into account the electric polarization produced in a liquid when density gradients are present. This is used to calculate the ultrasonic vibration potentials in an electrolyte, thus generalizing the Debye formula for this effect, to the case where the pure solvent produces its own potentials. The formula is in agreement with the few available experimental observations for reasonable values of the parameters. A calculation is also made of the disturbing effect of the measuring electrodes, and a simple equivalent circuit is derived under certain reasonably general conditions, from which the effect of an arbitrary impedance of the measuring apparatus can be allowed for.

Propagation of an ultrasonic wave through an absorbing liquid contained in a tube gives rise to a gradient of pressure along the axis which is determined by observing the velocity of flow of the liquid through a side tube connected in parallel with the main tube. The intensity of the ultrasonic wave is found by recording the voltage applied to the exciting transducer, the parameters of which are measured in the vicinity of resonance. Obstruction of the sound field by the intensity measurement is thus avoided. The observations are used to evaluate the absorption coefficients of liquids over the range 130 kc/s to 1560 kc/s.

The limits and accuracy of the technique are discussed and a new theoretical approach enables the pressure gradient to be derived from second-order acoustic variables without recourse to the concept of radiation pressure.

Some simple formulae are derived, which are useful in investigating wave propagation in a variable stratified medium. It is shown that the rays in a two-dimensional medium with slowly varying thickness and structure may be described by a characteristic time. From this time the ray angles at any horizontal position may be calculated. The characteristic time formula is also derived from considerations of normal modes, and then from considerations of energy flux. The formula still applies when there are slow bends or twists in the medium, with no limits on final amplitude. The extension to three-dimensional guides is discussed. Some formulae for intensity in slowly varying media are also obtained. The ideas are applied to the propagation of sound (especially underwater), shear wave, and electromagnetic radiation, and the present limitations of the method indicated.

Using good diamond counters and variable electrode separations from 10μ up to 1 mm, a survey of the surfaces of several specimens allowed the selection of crystals with the most uniform behaviour. The variation of the characteristics of the (pulse height, field) curve with electrode separation, could not be explained for these crystals simply in terms of trapping distribution or field distortion due to dark current. There is evidence that recombination of charge carriers at recombination centres close to the site of ionization is the dominating feature and dislocations are probably the defects responsible.

The thermoelectric power of a p-type semiconducting diamond has been measured in the temperature range 220°K to 700°K. After subtracting an electronic component, estimated from the previous Hall effect measurements on the sample, it has been possible to determine the phonon-drag component of the thermoelectric power. This component has been found to vary with temperature approximately as T^-3.6, with a value of about 2.5 mv deg^-1 at room temperature. The magnitude of the effect is of the order to be expected from the known behaviour of germanium and silicon.

Formulae for the Hall, magnetoresistance, photoelectromagnetic and Dember effects due to electrons and slow and fast holes are derived for the cases of energy independent relaxation time and lattice scattering. The expressions given are evaluated as far as the third order in magnetic fields for small magnetic fields and as far as the first order for large magnetic fields. They readily reduce to formulae which agree with those expressions which have so far been published. The results are discussed for the particular case of germanium.

A mathematical method is presented which leads in a direct manner both to an explicit expression for the frequency distribution of a disordered chain of atoms and to a powerful technique for the numerical evaluation of the spectrum. The application of the method to a model of a binary alloy is then considered.

The magnetic susceptibility of Ag-Mn solid solutions containing between 4.44 and 38.1 at.% Mn has been measured between 100 and 500°K. Alloys containing less than 20 at.% Mn obey a Curie-Weiss law and the p_eff values derived for the Mn atoms are discussed and compared with other published values for this system. Over the range 7-20 at.% the (p_eff, Mn concentration) curve is linear, indicating a 3d⁵4s² configuration for the Mn atoms at infinite dilution. Our own and some other published values for more dilute alloys are however not in good agreement with a linear extrapolation. Examination of published data shows that a similar discrepancy between dilute and more concentrated alloys exists in Cu-Mn solid solutions. No explanation of such an effect can yet be advanced, but the sensitivity of the p_eff values to the diamagnetic correction in very dilute alloys is pointed out, as well as a difference between electronic configurations deduced from susceptibility and from electron spin resonance measurements. Beyond 20 at.% Mn the Ag-Mn alloys no longer obey a Curie-Weiss law and the possibility of short-range magnetic order in them is discussed. Some comparisons are drawn between the solid solutions of Mn in Cu, Ag and Au.

In general terms a qualitative account of the extraordinary Hall effect in ferromagnetic media may be based on spin-orbit coupling. The magnitude of the effect produced is, classically, very sensitive to the actual path of the electron, or, quantum-mechanically, to some of the detail of its wave function. Here conventional quantum-mechanical transport theory is used in some quantitative detail to evaluate the actual magnitude of the extraordinary Hall coefficient R_s. The initial states of electrons after collisions with the lattice are assumed to be a superposition of Bloch functions with random phases. Bloch functions for the s- and d-bands of nickel are chosen in sufficient detail to make possible a numerical calculation. It is found that R_s/ρ², where ρ is the resistivity, increases very slightly for the pure metal with increasing temperature. At 290°K, R_s is found to be slightly less than -2 × 10^-12 ohm cm gauss^-1.

The presence of a large relaxation polarization in a number of metal oxides is reported. It is shown that this phenomenon cannot be explained in terms of ferroelectricity or in terms of the Maxwell-Wagner effect. A series of experiments are described from the results of which it is deduced that the materials behave as solid state secondary cells. It is suggested that oxygen vacancies act as charge carriers and interact with the metal electrodes deposited on the specimen.

Theoretical and experimental values of electric dipole oscillator strengths are used to calculate the coefficients of the leading terms in the series representations of the long range interactions between pairs of atoms. Values are obtained for pairs selected from the alkali metals, the inert gases, H, H^-, He⁺ and Li⁺ and are then employed to assess the accuracy of various approximate formulae. Where possible, comparison is made with experimental data and it is shown that the polarizabilities derived from molecular beam measurements are unreliable. A discussion is given of the measurements of elastic scattering of the alkali metals by the inert gases and it is concluded that the normal van der Waals interaction does not suffice to explain the observations at very low scattering angles.

An interferometric method is described which measures the frequency response characteristics of a lens, with automatic recording of the curves. The theory of the method is given, followed by a discussion of the problems arising in the design of the instrument. A new type of interferometer was devised, having very stable adjustment and permitting accurate control of the path length and relative lateral shear of the two interfering wave fronts. Using a double beam shearing interferometer, with corner cube reflectors, it was also possible to measure the spatial phase shift term of the complex frequency response and record this automatically.

The frequency response curves for a well-corrected lens in different focal planes have been compared with results obtained by Hopkins theoretically.

The influence of primary spherical aberration on the frequency response has been studied in certain cases and the results of theoretical calculations verified.

The effects of variations of the thickness of the individual layers of an interference filter made with dielectric multilayers are examined, with respect both to the displacement of the pass band and to its width as affected by random fluctuations in these thicknesses. It is shown that the use of special multilayer stacks having a large dispersion of the phase change on reflection can yield a much greater resolution only in the case of perfectly uniform films. The insensitivity of the pass band to variations of the spacer layer is compensated by a correspondingly large sensitivity to variations of the layers constituting the `mirrors'. It is also shown that the effect of random thickness variations is a minimum when the sensitivities of all layers are proportional to their thicknesses. Sample calculations and experimental illustrations are given.

The vapour pressure P of solid and liquid mixtures of argon and krypton of a number of different compositions has been measured over a temperature (T) range which includes the `melting' and `freezing' points of the mixtures. The solid-liquid phase-equilibrium diagram has been deduced by observation of discontinuities in slope of (log P, 1/T) graphs. The measurements show that argon and krypton are completely soluble in the liquid and solid states and that the solid solutions exhibit a positive deviation from ideality.

The solid solution data are compared with theory. Although the experimental results are not in complete agreement with any of the theories considered, the cell theory predicts approximately the observed magnitude of the deviations from ideality of the vapour pressure.

The energy levels of ²⁶Al have been investigated by magnetic analysis using the reactions ²⁷Al(³He, α)²⁶Al and ²⁴Mg(³He, p)²⁶Al at an incident energy of 5.8 MeV. The positions of 70 excited states of ²⁶Al are reported below an excitation energy of 6.865 MeV. No evidence was found for a doublet at 2.08 MeV.

It is shown theoretically that a cathode spot may be maintained on an isolated metal surface immersed in the plasma of a gas discharge. Two experiments are described in which arcs of this kind were produced on a mercury surface.