Journal of Scientific Instruments

The basic principles of resistance strain gauges are described and recent developments in the fields of semiconductor strain gauges and high-temperature strain gauges are discussed. Semiconductor strain gauges are much more sensitive than the metal ones commonly used but special circuit arrangements are often necessary if high accuracy is to be retained. Techniques for the measurement of static strains up to about 400°C are well established. The introduction of two new strain gauge alloys offers the prospect of increasing this range to about 1000°C.

A table of data gives percentage relative humidities over saturated solutions of eighty-eight different salts or salt mixtures. Remarks on the suitability of these solutions for humidity control are included.

Slight optical non-homogeneities in certain transparent specimens are usually rendered visible by putting the specimen out of focus. One then sees not the non-homogeneities themselves, but the focal spots and lines they produce. Zernike's method of phase-contrast illumination, on the other hand, provides the maximum contrast when the specimen is accurately in focus, so that what is seen bears a closer resemblance to what is "really there". A simple method of adapting an ordinary microscope for phase-contrast is described, together with the preparation of a test-slide to check its performance. The method has proved helpful in studies of the interaction between leucocytes and mobile organisms.

A method is described for visual observation of the output mode of a CO₂ laser. The method is that of controlled incandescence and the patterns produced can be dynamically viewed and are also easily photographed.

A new and sensitive type of infra-red radiation pyrometer has been designed and constructed, one form of which is specifically designed for measuring the temperature of aluminium extrusions in the 400-500°C range, with reduced dependence on the surface emissivity variations of the material. The basic instrument has two novel features. Firstly, it uses optical negative feedback from a semiconductor lamp to its lead sulphide photocell detector in order to give it overall stability, and secondly it uses an efficient infra-red cut-off filter, a water cell, to improve the power law relationship of received energy to temperature, so reducing the effects of emissivity variation. For the temperature measurement of aluminium and its alloys these variations are reduced to about half those encountered with a normal lead sulphide photocell pyrometer. The basic pyrometer is adaptable from narrow to wide angle working (0.75 in. viewing diameter at 7 ft, upwards) and will cover a range of temperatures from less than 300°C upwards, depending on the optical system. Without water cell filtering the minimum measuring temperature on low emissivity aluminium is less than 70°C.

The results of two-channel multiplier life tests in ultra-high vacuum are presented. In both cases the mean charge output was found to decrease progressively and to vary approximately as the reciprocal of the total number of counts for counts in excess of 10¹⁰. A mechanism for this decrease is suggested.

The confusion in the paper by Klein and Verheyden resulting from the use of the term `quartz' for vitreous silica in addition to its correct use for a crystalline phase of silica is briefly discussed and the differences in thermal expansion characteristics of the two materials are indicated.

All the principal techniques of flow visualization in water are reviewed, consideration also being given to the possibility of obtaining quantitative information from them. Static methods illustrate the pattern of velocity gradient at the surface of a solid boundary and are particularly valuable in indicating transition and separation of the boundary layer. Many kinetic methods involve the observation of tracer particles, either solid or bubbles of another fluid; the associated hydrodynamic and optical problems are discussed. Some of these methods may be adapted to indicate three-dimensional flow. Techniques based on streaming birefringence are reviewed but, for quantitative results, are found wanting. Chemical methods largely avoid the hydrodynamic problems but often have practical drawbacks. Among electrolytic methods that using hydrogen bubbles is remarkably versatile and now seems supreme; this technique is therefore discussed in some detail.

Infra-red detectors can be divided into two types: those which depend upon heating effects produced by the absorbed radiation (thermal detectors) and those which make use of photoconductive effects. The characteristics of the more important members of both types are discussed. The factors which determine their performance and which affect the choice of the most suitable type for a particular application are considered. Future trends in the development of infra-red detectors are briefly mentioned.

The history of the development of solid state devices for nuclear radiation detection is traced from the crystal counter to the latest semiconductor detectors. The physical processes which are involved in the detector are then outlined, and the features which are desirable are discussed. Some commonly used procedures for detector fabrication are described, and attention is drawn to certain problems which remain. Finally, some recent and novel applications of these detectors are briefly described.

The properties of katharometers are described in relation to the analyses of binary and multi-component gas mixtures. The effects of external conditions on accuracy and the factors to be considered in practical applications are discussed.

The most frequently used experimental techniques for the observation of the Mössbauer effect are reviewed. The most versatile and most widely used system consists of a negative-feedback controlled electromechanical drive used in conjunction with a multichannel pulse-height analyser, and particular attention is given to the problems encountered in the design of this type of equipment.