Table of contents

An apparatus is described which records continuously on photographic paper the extension-against-time curve of a metal wire creeping under stress. The length and time scales are recorded on the paper at the same time as the extension, so that deformation of the paper during development and drying does not affect the accuracy of the record, which allows an extension of up to 10 cm. to be read to within 0.02 mm. The apparatus also automatically removes and restores, repeatedly if desired, the load at times which can be set before the start of the experiment.

A method of determining the stress-strain relation of materials when stresses are applied for times of the order of 20 microseconds is described. The apparatus employed was a modification of the Hopkinson pressure bar, and detonators were used to produce large transient stresses. Thin specimens of rubbers, plastics and metals were investigated and the compressions produced were as high as 20% with the softer materials. It was found that whilst Perspex recovered almost as soon as the stress was removed, rubbers and polythene showed delayed recovery, and copper and lead showed irrecoverable flow. The phenomenon of delayed recovery is discussed in terms of the theory of mechanical relaxation and memory effects in the material.

The preliminary work on the measurement of velocity of propagation and attenuation of longitudinal sound oscillations in high polymer filaments already described by Hillier and Kolsky has been continued. It was concluded in that paper that temperature control would be advantageous, and that by varying the temperature and the frequency of oscillations more useful information could be acquired. An apparatus is described whereby the velocity of propagation and the attenuation of longitudinal oscillations throughout the range 0° to 50° c. and 500 c/s. to 30 kc/s. can be measured. The results obtained with one material, polythene, are discussed in terms of several theories of the elasticity of high polymers, and the constants of the several equations considered are calculated from the data.

The theory of boundary waves (Young, Rubinowicz, and others) is developed for the two-dimensional case and applied to discuss image formation for small apertures, obstacles and phase-retarding laminae. The theoretical predictions are illustrated by diffraction photographs. It is hoped thus to contribute towards a fuller understanding of microscope images for bright-field, dark-field and phase-contrast conditions.

The wave-front aberrations which occur in a pencil traversing a symmetrical optical system at a finite field angle are defined in a new way. Refraction and transfer formulae are derived for all the aberration coefficients up to and including those depending on the fourth power of the aperture of the pencil.

Measurements have been carried out on the spectral distribution of the photoconductive effect in layers of lead sulphide, selenide and telluride, at temperatures ranging from room temperature to that of liquid hydrogen. It is shown that cooling has a marked effect on the long-wave limit of sensitivity, which increases by as much as two microns on cooling with liquid hydrogen.

Brief descriptions of spectral measurements on these materials have already been published by the author. It is shown that depopulation of energy levels due to thermal activation of electrons is not responsible for the shift of spectral sensitivity. The suggestion is made that the shift may be related to the variation of dielectric constant with temperature.

Corrections to 1949 Proc. Phys. Soc. B 62 597