Table of contents

The author has been able to measure conductivity currents in hexane up to the breakdown stress by using a conditioning process which enables a stable state of the electrodes and liquid to be kept for many hours. It is shown that the results are consistent with a theory of electron emission from the cathode, and that the dominant process is the Schottky one, together with the mathematically similar barrier peak tunnelling component. The work function is found to be 1.0 to 1.2 eV, although this may contain a bulk liquid component. For stresses of 1 MV cm^-1 and greater, evidence is given of an ionization process within the liquid. Due to the difficulties of analysis, caused by the use of spherical electrodes, no estimate of the ionization increment factor α can be given. The tests give no information on the possibility of transient changes in the cathode state due to its inability rapidly to neutralize ionized particles reaching it.

Experiments are described on contact resistance effects in graphite powders: (i) as these are progressively compressed up to 3600 kg cm^-2 in the cold: this produces a marked decrease of electrical resistance, though it seems unlikely that much compression of the actual crystal lattice of graphite is effected; (ii) when loosely pressed powder is subjected to flash discharges from a bank of charged condensers (`flash sintering'): this is in order to explore possible mechanisms of sintering that avoid any prolonged heating of the graphite at high temperatures.

In the case of powdered polycrystalline graphite, both (i) and (ii) lead to a marked lowering and stabilization of the electrical resistance. Their influence on the thermoelectric power is discussed. `Flash sintering' promises to be a useful and accessible technique in studying the conductance of crystal compounds of graphite. The results throw a significant light on the nature of the conductance barriers in graphite powders.

Measurements have been made of the dielectric properties of methyl and ethyl alcohols in the wavelength range 3 cm-52 cm. It is concluded that the dielectric behaviour of both alcohols can be described in terms of a principal dispersion region at centimetre wavelengths together with a subsidiary dispersion region occurring at millimetre wavelengths.

A combination of two aspherical mirrors is considered which is aplanatic (i.e. axially stigmatic and satisfying the exact sine criterion). The shapes of the mirrors are determined analytically in finite form. The Schwarzschild-Chretien aplanatic telescope follows as a special case when one focus is at infinity.

An apparatus has been built for the measurement of mechanical stress in vacuum deposited films. A study of the stress changes during pulse annealing has been made with copper films deposited at several substrate temperatures between -150°C and +75°C. The annealing spectrum shows a prominent peak centred near room temperature which matches a resistance annealing peak in similar films and also in cold worked copper. The fall of tensile stress in this region suggests a vacancy mechanism. The theory of Murbach and Wilman for the behaviour of initial stress with temperature is not supported by these results.

Radar observations, on a wavelength of 50 cm, of inter-stroke lightning processes provide the following information on the nature of the charged column and the manner in which it is discharged:

(a) Inter-stroke junction streamer activity is most intense and most prolonged in the lower regions, that is, at heights ranging from 4-7 km. (b) Vertical development of the junction streamers may on occasion take place steadily to a height of about 10 kilometres. The velocities are in good agreement with those derived by Malan and Schonland. The junction streamer activity is somewhat less intense in the upper regions and tends to disappear while activity still persists in the lower regions. (c) the lower height limit of junction streamer activity remains at a more or less constant height throughout the flash. (d) The horizontal extent of the junction streamer activity in the lower regions is frequently of the order of 1½-2 kilometres. (e) The echoing properties of the junction streamers actually decrease during the latter part of the inter-stroke period and a further stroke does not occur until this decrease has taken place. (f) A stroke may take place from one or more regions in the charged column, without affecting appreciably the junction streamer process in another region which will nevertheless contribute to a later stroke in the same lightning discharge.

Modifications to the tentative description of the inter-stroke processes given by Malan and Schonland are suggested which would explain these results.

Some information has been gained on the nature and intensity of the noise radiated by lightning at 600 Mc/s. In particular it appears that at this frequency the noise is radiated from comparatively localized sources and may possibly be associated with the stepped and dart leader processes.

The nuclear magnetic resonance spectrum is calculated for an assembly of isotropically oriented systems of three identical nuclei of spin number ½ situated at the corners of an isosceles triangle. The theoretical spectra for triangles departing in various degrees from equilateral shape are compared with those recorded by Richards and Smith for the monohydrates of nitric and perchloric acids in polycrystalline form. It is concluded that if the oxonium ion in these acid hydrates is not equilateral, it does not depart from that shape by more than 10%. Interionic broadening is examined and it is concluded that for the acid hydrates a Gaussian function gives a good description of this broadening.

Using a substitution bridge method, the reverse current-voltage and capacitance-voltage characteristics of metal rectifiers have been measured at room and liquid air temperatures. At low temperatures the capacitance of Cu₂O and Se rectifiers, and the reverse current of the former, decrease, but that of the latter increases. The capacitance-voltage characteristics seem to be in good agreement with Landsberg's heterogeneous barrier layer theory, and the comparison of the experimental results with this theory gives effective dielectric constants of 4.3 and 2.0~4.3 for the barrier materials of Cu₂O and Se rectifiers, respectively.