Table of contents

For the first time bulk photo-effects have been observed in single crystals of lead sulphide. Both photoelectromagnetic and photoconductive measurements have been made, and carrier lifetimes and quantum efficiencies estimated therefrom. Quantum efficiencies are near unity for all samples, while lifetimes are found to range from 6 × 10^-10 to 9 × 10^-6 second. Measurements on lifetimes by two other methods give values in reasonable accord with the photoelectromagnetic values. Study of the spectral distribution of sensitivity establishes that there is no appreciable change in quantum efficiency with wavelength except for the rapid fall at 3μ, thus establishing that photoelectrically there is no difference between the main absorption band and the `tail' band. The optical activation energy (i.e. quantum energy at which sensitivity is halved) is 0.41 eV, and it is concluded that the thermal activation energy is very close to this value.

Errors are pointed out in analyses by Hall and Williamson and by Weiss of extinction in powders. Formulae are proposed giving a first-order correction for the reduction of reflecting power of small spherical crystals due to primary and secondary extinction. Neither formula accounts adequately for differences between observed and calculated intensities in Hall and Williamson's measurements on copper and aluminium.

Counter spectrometers are described for the accurate semi-automatic measurement of diffracted x-ray intensity over a range of scattering angles from 0° to 130°. A proportional counter is used in conjunction with a linear amplifier and a single-channel pulse analyser. The degree of monochromatization attained with a copper target and nickel filter is such that 99% of the measured intensity is due to Cu Kα radiation. The various corrections to be applied are discussed.

Measurements have been made of the excess noise produced by a point-contact germanium rectifier in the frequency range 0.117 c/s to 14 Mc/s at 29.5°C, with the reverse direct current I as parameter. The experimental noise spectral density is shown to consist of three well-defined types of component: (a) a basic component ∝ f^-1 detectable over as many as seven decades of frequency, (b) two components ∝ [1 + (2πfτ)²]^-1 associated with single relaxation times τ=τ₁ and τ₂, (c) a uniform component detectable at high frequencies. There is no simple dependence of these densities on current, except that the uniform component is equal to shot noise for small currents. τ₁ and τ₂ are found to be dependent on I.

This paper is concerned with the explanation of the spread F echoes often obtained on frequency-sweep records of equivalent height in the ionosphere, especially at night and in the auroral regions. It is shown in the Appendix that the production of a mountain range of echoes from a single incident pulse can be accounted for if there is a periodic structure of ionization in the F layer. There are two mechanisms of ionization in the ionosphere, namely the incidence of ultra-violet radiation from the sun, and the influx of a stream of charged particles that are deflected by the earth's magnetic field towards the geomagnetic poles. The ultra-violet light produces well defined layers, the E, F₁ and F₂ layers, with a single maximum of ionic density within them, and they may be satisfactorily explained by Chapman's theory, taking into account the recombination process but neglecting diffusion effects. In the case of particle ionization, however, the effect of diffusion becomes predominant. It is shown in the paper that the inclusion of the diffusion term in the differential equation of layer formation leads to the possibility of a periodic structure within the F layer of the kind needed to explain the spread echoes. This is because the differential equation has as its solution a doubly periodic elliptic function that provides the required periodicity of ionic density as a function of height. The form of the mathematics gives the essentially periodic nature of the solution without any detailed appeal to the initial conditions, but reference is made to the auroral curtains indicating the existence of sources of charged particles that maintain the periodic structure of ionization in between the curtains. Whereas the ultra-violet light causes layers to be formed that are many wavelengths thick, the particle ionization causes very thin layers only two or three wavelengths thick embedded in the background of ionization.

The character of jet noise undergoes a marked change above choking, the noise due to turbulent mixing being dominated by a powerful whistle or screech whose wavelength is related to the regular shock wave spacing. The mechanism in two-dimensional flow is further examined (by the aid of a dynamic Schlieren apparatus), verifying the suggested mechanism and showing the similarity to that in axially symmetric flow where discontinuities in frequency, partly analogous to edge tones, occur. The resultant sound emitted as the periodic eddy system traverses the regular shock wave pattern is highly directional, producing a powerful beam at doubled frequency normal to the jet and an intense beam at eddy frequency in the upstream direction adjacent to the jet, resulting in fluctuations in jet velocity direction at the orifice which initiate new stream disturbances.

A gain criterion for the self-maintained cycle is given, enabling certain qualitative deductions concerning the intensity to be made, and use will be made of this in considering methods of reducing the noise level.

The penetration of electrons of energies between 10 and 40 kev into a number of phosphors, and the extent of their scattering, is measured on microphotographs of the luminous region produced in the phosphor by a beam of electrons of less than 0.75 μ diameter. A description is given of the apparatus and of the electron optical system designed to produce the fine focal spot. The phosphors used in the experiment were single crystals of the thallium-activated iodides of K, Rb and Cs, and of the tungstates of Ca and Cd, and a luminescent plastic. An attempt is made to account for the discrepancies in the electron ranges as found in the present experiments compared with those obtained by other methods. An estimate is made of the distribution in depth of the energy dissipation in a phosphor when bombarded by a beam of electrons.

The authors have studied theoretically the water bell phenomena, interest in which was revived recently by Hopwood. A numerical method of solution of the differential equation of the bell's surface is described and the results obtained are given in the form of graphs. It is shown that the shapes depend on three parameters and, in order to determine numerical magnitudes of these, experiments similar to Hopwood's were performed, during the course of which some photographs of the bells were obtained. Four of these are included. The paper ends with a section comparing the theoretical shapes with those obtained experimentally showing the agreement between the two to be good.

The thresholds are determined for the incidence of electron absorption, decomposition and secondary emission for several types of thin film bombarded with slow electrons. With crystalline barium oxide, electron absorption and decomposition both set in at a bombarding energy near 4 eV, and are intensified at 7 eV. Secondary emission is initiated at 5 eV and increases at 7 eV and 10 eV. The optical absorption threshold is at 3.8 eV, and is attributed to exciton formation; it seems likely that the electron absorption and the decomposition are also due to exciton formation. With oxidized films of barium, crystalline barium oxide is not formed; the absorption and decomposition thresholds are at 7 eV, and the main secondary emission threshold at 10 eV. Decomposition of barium chloride sets in at 5 eV, and of barium sulphate at 7 eV. With films of the alkali halides, the decomposition is intensified at an energy corresponding with the first optical absorption band, having been initiated at a lower energy. A tentative explanation of this is proposed.

A systematic investigation has been made of the currents drawn to a metal cylinder surrounding a concentric high-current arc in hydrogen in the presence of a longitudinal magnetic field. Characteristic curves show currents drawn with various positive and negative potentials applied to the cylinder. The shape of the curves is discussed qualitatively in terms of the radial motion of charged particles in the electric and magnetic fields. Consideration is given to the possibility that the action of the arc current, with the magnetic field and electric field due to the cylinder, might produce an appreciable evacuation of gas molecules from the region around the arc column.

A high-resolution image may be derived from a large number of `dark-field' images of normal resolution, obtained by varying the angle of incidence of the electron beam in a standard electron microscope. The intensity distribution in each dark-field image is multiplied by an appropriate cosine function. Summing gives the function, for one dimension, C(x) = ∫ V(X) G(-X) V(X + x) G(x - X) dX where V(x) is the potential distribution in the specimen and G(x) defines the loss of resolution due to lens imperfections in normal micrographs. From this, V(x) may be derived. The practical applications of the method are limited by requirements of specimen and instrumental stability.

Measurements of the electrical resistivity of copper-iron alloys, in the annealed condition, have shown that the concept of `coherent' and `incoherent' precipitate can explain the observed features in the resistivity curve.

Cold working of the alloys has been shown to result in resistivity changes which are compatible with the idea of `locking-in' of dislocation lines about the precipitated particles of iron. Calculation from x-ray observations of the distance apart of these particles gives a figure of approximately 10^-5 cm. This is quite close to that estimated from Orowan's equation for the distance between precipitate particles most effective for locking-in of dislocations in an age-hardened alloy.

Estimates of the number of vacancies present in a 95% cold worked alloy suggest that vacancies alone will not account for the observed resistivity increases. At high degrees of cold working, dislocations are probably the predominant factor in increasing resistance.