Table of contents

Recent developments in solid state image devices using photoconductors and electroluminescent phosphors are described. The simplest device uses a thin homogeneous phosphor layer between transparent electrodes while more complex amplifiers use a combination of photoconductor and phosphor layers. By using alternating applied fields, microcrystalline photoconductors and phosphors dispersed in dielectric films may be used. The solid state amplifier offers possibilities for image conversion in the infra-red up to 6 μ, in the ultra-violet and in the X-ray regions.

Recent work on semi-conductors has led to materials in which the Hall effect in the earth's magnetic field is strong enough to form the basis of a simple electrical compass. The theory of the device is briefly presented and expressions derived for the voltage and power output, and the ultimate sensitivity obtainable. The comparative merits of various semi-conductors are shown in tabular form; this information should be useful also for other devices based on the Hall effect.

An experimental compass unit is described, which had a power output of 6 × 10^-9 W/degree of arc. Various possible systems using such a unit are briefly discussed, including the use of super-regeneration for increasing the power available.

The thermal comparator described consists of two metal balls similarly mounted in a block of balsa wood, but one is at a slightly lower level so that it touches any surface on which the block rests. After heating to a small fixed temperature excess the block is laid in contact with the test surface. Differentially connected thermocouples attached to each ball measure the increased rate of cooling of the ball which makes contact. The differential e.m.f. is observed 10 s after contact is made and is shown to be a function of the thermal conductivity of the material on which the ball rests. When calibrated in terms of at least two substances of known thermal conductivity, determinations of thermal conductivity can be quickly made. The accuracy from single readings made on several alloys of iron, aluminium and magnesium is about ±6%, but lead departs considerably, due, it is thought, to its softness.

The condition of the surface affects the readings, so the calibration should be made for a similar surface condition. Alternatively, the device can be used as a means of measuring surface finish. Other possible uses are for measuring the thickness of surface deposits and for non-destructive sorting or identification of solid materials.

An infra-red Fabry-Pérot interferometer working at very low orders as a tunable narrow band interference filter is described. The high reflecting surfaces are provided by germanium/cryolite quarter-wave stacks and the air gap between the surfaces may be varied to change the wavelength passed by the filter. It is shown that the luminosity of such an interference monochromator may be several hundred times higher than that of a prism or grating instrument of comparable size.

A method for the determination of water in several gases is outlined in which use is made of the absorption bands of the water molecule in the 1200 Å region. A 20 Mc/s electrodeless discharge in hydrogen excites the 1216 Å Lyman α line very strongly and relatively free from neighbouring lines and bands. The radiation passes through the absorption cell, closed at both ends by lithium fluoride windows, on to the tungsten cathode of a photomultiplier which is insensitive to light of wavelength longer than 1400 Å.

The paper details the conversion of an existing isothermal bomb calorimeter to an adiabatic calorimeter with automatic control. Thermistors in the inner and outer vessels are included in two arms of an a.c. Wheatstone bridge: any rise in temperature of the inner vessel above that of the outer vessel results in phase reversal of the output voltage from the bridge, which operates a gas-filled tetrode controlling a 500 W heater in the outer vessel. A second tetrode, operated by off-balance voltage, controls a 1000 W heater in the outer vessel which accommodates the initial rapid temperature rise when the bomb is fired. The equipment has been found to be more rapid and much less tedious for routine use than the usual isothermal method. Determinations of heat content take approximately thirty minutes: the coefficient of variation of a single determination, with an average temperature rise of 1.9° C, is 0.34%.

A lateral extensometer is described by which the lateral strain occurring in a cylindrical rock specimen subjected to a linear axial compressive load in a compression test, can be read or recorded continuously up to the point of fracture of the specimen. The lateral extensometer has been designed to operate in conjunction with a compressometer (described in a previous article) which measures the axial strain occurring in the specimen. The measuring element used in the lateral extensometer is a linear variable differential transformer and deformations of the specimen of the order of 1 μin. can be measured. The instrument has been designed in such a way that it suffers no physical damage when the specimen, to which it is attached, fractures. In fact, over 300 rock specimens have been tested using one of these instruments without its accuracy or mechanical condition having been seriously impaired.

Values are given for the electronic circuit required to detect, amplify and rectify a variation in resistance produced by fairly rapidly changing water levels on a pair of immersed probes, a situation met in the measurement of water waves in laboratory experiments. The accuracy of such an apparatus is checked and assessed by comparing with the usual pointer type of gauge instead of by further electrical means, and is well within the usual requirements for such experimental work.

A photographic technique is described that affords data on several phenomena associated with freely falling drops. An essential feature of the technique is a two-camera shutter designed to obtain photographs of the falling drop at two fall distances. Velocities are calculated from a consideration of displacement and time data that appear on the photographs. Information regarding the dynamics of drop deformation is inferred from the geometry of the image pairs and the time interval between photographs. The time interval is dependent on the dimensions and speed of the shutter. The possible error in the calculated velocities of large drops is 2.5%.