Table of contents

An attempt is made to formulate a theory of irreversible processes in fluids with the same degree of completeness and rigour as the theory of equilibrium based on statistical mechanics. The theory of the grand partition function is generalized to systems not in thermodynamic equilibrium, and to determine the distribution functions an equation is derived which is strictly analogous to Boltzmann's equation in the theory of rare gases. A definition is given of the entropy of a system not in thermodynamic equilibrium, which is more satisfactory than others previously proposed. An H-theorem is proved to establish the natural increase of the entropy so defined.

The proton magnetic resonance absorption in polyisobutylene has been measured over the temperature range - 196°C to 90°C. The unusually large width of the absorption line at the lowest temperatures is interpreted as largely due to the well known close approach of the methyl groups on alternate carbon atoms in the chain. The usual reduction in line width associated with reorientation of methyl groups about their C₃ axis is notably absent and supports the contention that the methyl groups are severely interlocked. A reduction in the second moment of the absorption line which sets in over a range of temperature near - 10°C is associated with chain motion. A satisfactory correlation has been obtained between this observation and measurements of mechanical properties. Experiments on PIB swelled with benzene substantiate the interpretation given above and suggest that valuable information as to the nature of the swelling process of rubber-like polymers may be obtained by means of nuclear resonance studies.

Attention is drawn to the importance of recognizing the various relaxation times associated with a single relaxation mechanism. The physical origin of the differences between the relaxation times is indicated and the relationships between them are illustrated by examples. A general formula is derived which relates the relaxation times for a single mechanism under different thermodynamic constraints. Connection is established between chemical rate constants and relaxation times. It is concluded that there is no intrinsic relaxation time derivable on a kinetic basis without reference to the thermodynamic conditions under which the relaxation takes place.

A comparison of the bearing and elevation angles of radio waves from two stations situated about 2000 km to the North and to the West of the receiving site yields results which are sometimes consistent with the existence during the daytime of effective ionospheric tilts which are similar at places more than 1000 km apart. These tilts are of the order of half a degree and are in opposite directions during the morning and the afternoon. They may correspond to actual tilts of a uniform ionosphere or be due to horizontal gradients of electron density. The factor relating the tilt angle to the observed bearing deviations was measured experimentally and the effective tilt angle found to be approximately twice the bearing deviation.

A direction finder which aligns itself along the horizontal projection of the wave normal of a radio wave is described. The system uses two pairs of aerials situated at the corners of a rectangle about 10 m by 5 m, the signals in each pair of aerials being combined in such a way that a required 90° phase shift is maintained accurately at all frequencies up to about 17 Mc/s. Sense determination is manual. The accuracy of such a system is discussed and it is shown that the aperture of a direction finder must be small if it is to follow bearing fluctuations when there is an appreciable spread of the component waves in the incoming signal.

The errors which occur in the measurement of low angles of elevation by centimetric radar on a flat ground site are estimated theoretically and compared with experimental measurements. It is demonstrated that a very considerable improvement in angular accuracy is obtained by erecting a screen to intercept the ground-reflected wave. The diffraction effects due to the screen are discussed.

The electrical resistivity and Hall coefficient of a semiconducting diamond (Type IIb) have been measured between - 100°C and 600°C using standard d.c. methods. The diamond behaves like a normal p-type semiconductor. At 20°C its resistivity is 270 ohm cm and the Hall mobility of the holes is 1550±150 cm² volt^-1 sec^-1. Near this temperature the mobility varies approximately as T^-3/2. The activation energy of the acceptors is 0.38 eV. It is suggested that these acceptors are impurity atoms and that the total concentration of impurities is smaller in Type IIb diamonds than in other diamonds.

The infra-red absorption spectrum of this diamond is typical of Type II diamonds but it shows an additional prominent peak at 3.57 μ, smaller peaks at 4.1 μ, 3.4 μ and 2.5 μ, and a continuum of absorption in the range 1 to 2.5 μ. This extra absorption is reversibly temperature dependent. The intensity decreases with increasing temperature and the peaks are undetectable above 300°C.

At - 155°C, photocurrents of the order of 10^-9 amp have been detected in the wavelength range 0.9 μ to 3.6 μ. A tentative model is proposed to relate this photoconductivity and the major absorption peak to the mechanism of conduction.

The spectral emissivities of iron, nickel and cobalt have been measured over a wavelength range 1.0μ to 3.0μ and at temperatures between 650° and 1350°C. The results show anomalies in the cases of iron and cobalt occurring at temperatures in the neighbourhood of the respective Curie points. Tables of the temperature coefficients of emissivity for the three metals are given.

The interaction between a rapidly oscillating electric field, such as is found associated with a light wave, and anisotropic molecules is discussed and it is shown that there will be a tendency for the molecules to be oriented. An isotropic medium will therefore become doubly refracting, the optic axis being parallel to the electric vector of the oscillating field, or in the case of normal light to its direction of propagation. For the birefringence to be easily measurable a very intense beam of light would be necessary, and a flash apparatus would probably be the most suitable source.

The intensity of light scattered at 90° to the incident beam has been investigated as a function of wavelength for a specimen of colourless, apparently perfect, quartz. The wavelength function indicates that, in addition to the light scattered by thermal fluctuations of density, to be expected in any crystal, however perfect, there is a contribution from some real-structure possessing a linear dimension of about 3000 Å. Previous work by the present author pointed a similar effect in NaCl crystals, in which a dimension of about 1500 Å was derived and a preliminary interpretation of the effect was offered in terms of a theoretic real-structure first suggested by Born and later discussed by Fürth. It is now pointed out that the formula given by Born does lead to a dimension for quartz about twice that for NaCl. But since more than one kind of imperfection could produce this `size effect' a discussion is given of imperfections from this point of view. Two kinds come into consideration: (i) clusters of the correct size randomly embedded in the lattice, (ii) a three-dimensional net throughout the lattice, made up of surfaces of disorder enclosing regions, of the correct size, perfection. It is argued that the former will produce discrete appearances the beam of light, detectable by ultra-microscope, and that an entirely diffuse beam of light indicates their absence. This is the case with several crystals which nevertheless exhibit the size effect, and it seems therefore that the latter alternative, the net, has to be accepted. This conclusion is discussed with reference to present knowledge of subgrain structure as well as to the theory of Born and Fürth.

A new experimental technique is described for measuring scintillation decay times in the millimicrosecond region. The method developed uses the fact that a pulsed current, as delivered by the photomultiplier scintillation counter produces a far greater power dissipation in a resistive load than does the same mean steady current. By measuring the power dissipated the pulse duration, and hence the scintillation decay time, may be found. A particular advantage of this method is that stochastic fluctuations of events in the scintillation counter may be neglected. These fluctuations give the multiplier output pulse a very irregular profile and are analysed in detail using statistical generating function.

The shapes and durations of scintillations excited in anthracene, trans-stilbene and para-terphenyl crystals are investigated. For anthracene the scintillation pulses delivered at the multiplier anode are exponential with decay times of 31 mμsec and 53 mμsec for electron and alpha particle excitation respectively. The shapes of these pulses are corrected for the effects of transit time spread in the multiplier and for the effect of self-absorption of fluorescence in the crystal to determine the true shape of the molecular scintillation. For both electron and alpha particle excitation this consists of a short initial spike of fluorescence followed by a long decay, non-exponential, component; the major part of the emission is contained in the long decay component. A qualitative analysis of the measurements for trans-stilbene and para-terphenyl shows that the scintillations from these crystals are also of this general form.

These results are interpreted in terms of the description proposed previously by the author for the scintillation process in organic crystals. This description predicts this shape for the molecular scintillation and is therefore supported by the present measurements.

Applications of moiré to surface strain measurement are described. A theory of moiré is developed, based on the concept of the covariant vector, which leads to a simple description of infinitesimal strain fields. Extension to the case of finite strain is considered, and parallels with crystallography are drawn.

Experimental results are discussed covering many aspects of the propagation and reflection of finite amplitude sound pulses and weak shock fronts. These are compared with theory and show amongst other features (a) that an N-shaped wave form tends to shorten on reflection and (b) that a shock front disappears on reflection at an open-ended tube.

The emission and excitation spectra of the luminescence produced in potassium and sodium chlorides by thermal shock, mechanical strain or by the introduction of the divalent ions Ba, Sr or Ca show similarities in the same host crystal which are explicable on the assumption that the positive ion vacancies comprise or are part of the luminescent centre. Differences in the emission spectra in the same host crystal activated in these various ways are attributed differences of association of the vacancies. The electrical conductivity of pure annealed material as determined by an a.c. method is increased by thermal shock or mechanical strain. The similarity of the gradients of the graphs shows log conductivity plotted against reciprocal temperature of the thermally or mechanically strained samples with those of the annealed samples containing added divalent ions shows that the increased conductivity is due to positive vacancies; the magnitude of the increase as compared with the pure annealed crystals indicates that the number of `free' vacancies is increased some four five times by the thermal shock.