Table of contents

From measurements of the characteristics of several natural p-n junctions the intrinsic resistivity of lead sulphide is calculated to be 3.1 Ω cm. Hence at room temperature the intrinsic carrier concentration is 2.7 × 10¹⁵ electrons or holes per cm³ and the width of the forbidden zone 0.40 eV.

Fluorescence excitation spectra are measured for some organic crystals using excitation wavelengths down to 2200 Å. The fluorescence quantum efficiency of crystalline p-terphenyl is found to be constant with wavelength of exciting radiation but apparent variations are found for trans-stilbene and particularly for anthracene crystals. These variations are due to surface escape of fluorescence the extent of which is sensitive to the depth of penetration of the exciting light in crystals which re-absorb their molecular fluorescence. The fluorescence excitation spectrum thus provides information about the absorption spectrum of the crystal. For excitation of the 001 crystal face of anthracene absorption maxima are found at 3960 Å, 3740 Å, 3540 Å, 3370 Å, 3220 Å; these wavelengths are rather longer than hitherto reported. The first fluorescence maximum lies at 3980 Å hence the 0-0 absorption and fluorescence transitions effectively coincide.

Measurements made with an anthracene crystal having a very thin surface quenching layer of low fluorescence efficiency show that migration of electronic excitation energy through the crystal lattice occurs initially by a short range and non-radiative process. Possible transfer mechanisms are discussed and it is concluded that the most probable process is a quantum-mechanical resonance exchange between adjacent molecules resulting in a random diffusion of the excitation energy through the lattice. This is followed by the secondary and trivial process of fluorescence emission and re-absorption.

The absolute photofluorescence quantum efficiencies of p-terphenyl and trans-stilbene crystals are measured by comparison with anthracene for which the quantum efficiency is 0.80 ± 0.05. At a temperature of 290°K the efficiencies of p-terphenyl and trans-stilbene crystals are respectively 0.52 and 0.65.

From microwave resonance measurements, g values and line widths for nickel and for Ni-Cu, Ni-Al, Ni-Sb and Ni-Mn alloys with high nickel content have been determined. Measurements were made at a wavelength of 1 22 cm and usually in the temperature range 20-200°C.

A constant temperature independent g value of 2.19 ± 0.02 was found for all specimens except the Ni-Mn alloys; for these a decrease in g value with increase in manganese content occurred, reaching 2.11 for the 13.5 atomic per cent Mn alloy. An explanation of this behaviour is given.

Absolute absorption measurements were made and the line widths are expressed in terms of the damping constant 1/T₂ introduced by Bloembergen (1950). For a given alloy the shape of the curve of 1/T₂ against reduced temperature is similar to that found by Bloembergen for nickel but varies from alloy to alloy. A single curve of exponential form is found to represent the results when 1 T₂ is plotted against saturation intensity of magnetization M, an increase in 1 T₂ accompanying a decrease in M. A possible explanation is qualitatively presented.

Coal specimens having weights within the range 10^-9-10³g were broken in crushing tests. The crushing loads were found to vary as the half power of specimen weight, instead of the two-thirds power predicted by simple dimensional analysis. A theory is proposed to account for these facts and this theory is used to provide an estimate of the rupture energy for one coal (Llandebie). The value of energy obtained, 3.0 × 10⁴ erg cm^-2, seems to indicate that plastic deformation is occurring in this coal which is normally considered completely elastic. The occurrence of plastic flow in very small particles of any coal under similar conditions of loading is a logical sequence of the theory. This plastic flow has been observed in two of the coals used in this work.

Electronic digital computers in Britain and the U.S.A. have been used for the tracing of rays at high speeds through optical systems. The larger machines have enough auxiliary storage, either magnetic drum or tape, to accommodate enormous amounts of information so that it is possible to carry out extensive numerical investigations concerning lens systems.

This paper deals with machine methods of minimizing the calculated geometrical aberrations of systems in much the same way as the optical designer using conventional desk machines. It will be shown that the main designer's task can be achieved automatically in a remarkably short time. The machine referred to is the Manchester Electronic Computer (MADAM), but the remarks apply to any high speed machine with a reasonable storage capacity. It is not proposed to enter into any of the programming details, these would require a separate paper.

It is shown that the focal lengths of all known iron shrouded magnetic electron lenses lie between two fairly close `limit-curves', if f is measured in the unit S+D for symmetrical or S+R<sub>1</sub>+R<sub>2</sub> for asymmetrical lenses of bore radii R<sub>1</sub> and R<sub>2</sub> and gap length S, f being plotted as function of (NI)<sup>2</sup>/V<sub>r</sub>. The lenses of the more usual S/D ratios (0.5 less-than-or-eq, slant S/D less-than-or-eq, slant 2) lie very close to the lower `limit-curve', i.e. all these lenses can be represented by a single focal length curve. A similar representation can be given for the other lens constants.

The field emission current from a simple semiconductor has been evaluated in a manner similar to that employed by Fowler and Nordheim for a metal. It is shown that special effects will be associated with the state of the semiconductor surface. The presence of an appreciable number of surface states should give very low currents until fields high enough to break down the internal barrier are applied. When this occurs, or if there are no surface states at all, the field will penetrate into the semiconductor and lead to a lowering of the conduction band edge. This causes degeneracy in the electron distribution near the surface region and a greatly enhanced emission not unlike that of a metal

It is pointed out that the current-field emission characteristic dominated by surface states should be strongly temperature dependent while that dominated by field penetration is not.

The limits of applicability for the various results are indicated.

Crystals of cadmium sulphide show new absorption bands when excited by visible light (0.546 μ). The bands extend from the excitation region out to 1.4 μ with a maximum intensity at 0.78 μ. The bands are stable in the dark at low temperatures but are thermally bleached on warming. They are also bleached by radiation of their own wavelength. Correlation is found between thermal bleaching characteristics and electrical conductivity characteristics. A model is proposed in which thermal bleaching and electrical conduction on warming result from the escape of trapped electrons while optical bleaching frees positive holes which migrate to traps and recombine with trapped electrons.

The relation between the isothermal and steady-state current-voltage characteristics of a device in which the current depends on both the voltage and the temperature is deduced quite generally without assuming electrical linearity or Newtonian cooling. The phenomena of voltage turnover in a device with a positive temperature coefficient of d.c. conductance, and of current turnover in a device with a negative coefficient are deduced.

The a.c. admittance for a small a.c. voltage superimposed on the d.c. voltage determining the operating point is derived in terms of the thermal admittance of the device, i.e. the complex ratio of the alternating components of power and temperature. For the special case in which the thermal admittance consists of a shunt combination of a fixed thermal capacity and thermal conductance the electrical admittance has been evaluated in detail. Three-element equivalent circuits (of two resistors and a capacitor or inductor) can represent the electrical admittance of such a device and its locus is semicircular; the reactive element represents the effect of thermal inertia and its nature is determined by the sign of the temperature coefficient of d.c. conductance.

Where heat loss from the device is by a path which is not `thermally short at the frequency under consideration the thermal admittance no longer has simple form but has to be determined from the diffusion equation. This is done for the case of one-dimensional heat conduction (e.g. along the supporting wires of a thermistor or a lamp) or for radial heat flow (e.g. from the contact in a point-contact rectifier). The general condition for the admittance locus still to be circular, but with the centre not on the conductance axis, is formulated.

An apparatus for the measurement of electroluminescence in single crystals of zinc sulphide under a range of conditions is described. The results of the measurements are compared with existing theories, which are shown to be unsatisfactory. Modified forms of these theories are developed which are in much better agreement with experiment.