Table of contents

It is less than three years since the first optical maser or laser was brought into operation, and spectacular progress continues to be made. Solid-state devices have not so far achieved the same spectral purity as their gaseous counterparts, but their performance as source of giant light pulses has made them ideal for experiments on non-linear optical effects, such as harmonic generation, which depend on quadratic or high powers of light intensity.

The coherent output of an optical maser is generated by much the same `feedback' principle as in a radio oscillator, but the dimensions of the optical resonator are such that it is a three-dimensional momentum or mode pattern whose build-up must be considered. Comparing coherent light from a laser with ordinary light from a thermal source, we find that the advantage of the former depends very much on the type of experiment, but can be twelve orders of magnitude where both brightness and spectral purity are important.

One of the main aims of the present work is to find new laser materials that can operate at new wavelengths and with low pumping power, but considerable attention is also being given to securing more nearly optimum performance from an existing material. Three of the main present limitations are divergence of the beam caused by variations in refractive index, frequency drift caused by the optical pump, and `spiking' or instability of light output. Recent progress towards solving these problems is reviewed.

The history of ultrasonics is outlined, to provide a background for the discussion of presentday applications. These can be classified as processing, testing and communications.

In processing applications, ultrasonic vibration at high power densities is used to produce a physical or chemical change in material. In liquids, the effective agent is often cavitation, which causes emulsification and dispersion, useful for cleaning and atomization.

Focused ultrasound is used for surgery. Echo sounding techniques are used for testing materials as well as at sea. They have recently been applied in medical diagnosis. Ultrasonography by image-forming methods may be an alternative method. The high frequency elastic properties of a material can be measured with great precision and give information about its structure.

The most important `communications' application is in ultrasonic delay lines, which are useful in radar and data-handling systems.

This paper discusses the experiments which have been and are being made, with the help of rockets and artificial satellites, to investigate phenomena outside the earth. These phenomena are discussed in order of increasing height. It is explained that the first objective is to get, effectively, outside the earth's atmosphere for the purpose of examining those solar radiations which are incapable of penetrating it. The next is to examine the nature and movements of the very tenuous outer atmosphere. And finally the objective is to explore interplanetary space and the phenomena which occur between the sun and the earth. Methods of experimenting are described in outline and some typical results discussed.

The instrument is primarily designed to measure the magnitude and direction of the magnetization of small rock samples. The samples in the form of cylinders (5 cm³) are rotated at 375 c/s in a freely suspended air turbine. The noise level is equivalent to a remanence of about 3 × 10^-7 e.m.u. cm^-3 (assuming a standard sample).

The mechanical and optical components are described of an integrating microspectrophotometer, which can either measure absorption spectra of specimens as small as 1 μm in diameter, or integrate the absorbances measured at one wavelength from any irregular specimen. Facilities are provided for making an outline drawing of any part of the specimen, which can then control the selection of areas to be integrated.

The electronic circuits include a high-speed electronic servo for maintaining the photomultiplier signal constant, logarithmic conversion and integrating elements. Various tests of the instrument are described.

A new oil-filled vacuum manometer is described, which has considerable advantages over conventional vacuum gauges. It is provided with a simple de-aeration chamber, in which the oil can be completely freed from dissolved gases within a few minutes, thus avoiding subsequent errors from the release of dissolved gases.

The new gauge compares favourably with a McLeod gauge of range 0-10 mmHg: it is much simpler and cheaper to construct; it is of similar accuracy (except at pressures below about 1 mmHg) and of much wider range; it gives readings continuously; and its accuracy is unimpaired by the presence of condensable vapour.

A thermoelectric unit is described for rapid and easy freeze-drying of biological tissues in vacuo. The design is simple and the whole process can be made automatic.

This article describes an electronically controlled recording balance for obtaining, concurrently with the differential thermal analysis record, the weight record of a sample during heating. The apparatus is used for the study and for the determination of the quantities, by weight, of calcareous and hydroxyl components in materials. The instrument is capable of determining quantitatively the weight of active materials to within 6% of the sample weight by measuring the area under the peak of the differential thermal analysis record and to within 0.5% from the weight record. Some results obtained are presented and discussed.

The theoretically evaluated electron energy distribution curve from a function of the type U exp ( - U/kT), as given by Fowler and Honig, for electrons emitted from a heated filament is compared with the experimentally determined curve. The shift in the maxima of the experimental curve as compared with the theoretical one is attributed to the contact potential in the system, and consequently this shift has been used as a means of evaluating the contact potential during the course of an experiment. The shape of the ionization efficiency curve is analysed and the linearly extrapolated value is shown to be lower than the true ionization potential by 2 kT when the ionizing electrons have a distribution corresponding to the abovementioned function. Finally the experimentally determined contact potential and the value 2 kT evaluated corresponding to the filament temperature are used to obtain the ionization potentials in four gases on an absolute scale to within ±0.08 ev. The results are compared with spectroscopic data.

A camera system is described in which the film advances by short, sudden jumps. Three similar cameras are described, in which the film is advanced 10, 100, and 150 times per minute respectively. The cameras were designed for use in meteor spectroscopy, but the technique should prove useful in other applications.

A current stabilizer is described for use with self-rectifying x-ray tubes where a three-phase supply is available. Fast response is obtained with a control ratio exceeding 100 by using hard valves in a pulse-operated circuit.

The stabilizer has been adapted to correct automatically for counting losses due to paralysis in the x-ray detector when integrating a diffraction profile. The corrected integrated intensities thus obtained were within 0.5% of calculated values.

A robust, strain-free mount for indium resistance thermometer wire is described, which is suitable for use down to 14° K. Two thermometers have been constructed which displayed ice-point resistance fluctuations of less than 1 part in 8000 and 1 part in 14000 respectively after repeated cooling to low temperatures.

X-rays monochromatized by a double-crystal spectrometer are incident on two counters. One counter measures the intensity of the primary beam, the other the intensity of the beam after transmission through an absorber. The x-ray tube need not be stabilized and long counting times can conveniently be employed to reduce statistical errors, since the apparatus is fully automatic.

A stable and linear negative feedback amplifier has been designed for feeding the output signals from very high impedance transducers, such as vacuum photocells, directly to a digital voltmeter. When this intermediate amplifier is used fluctuations in the input resistance of the voltmeter are unimportant, whilst the transducer may be moved during the measurement without causing spurious signals due to the generation of noise in the connecting cables.

An instrument is described which enables a fibre or filament to be rotated under a microscope so that the fibre profile diameter at any angle may be measured. In particular, for elliptically shaped fibres such as wool, the major and minor axes and the angles at which they occur may be observed. The instrument uses a dial gauge, which measures in 0.0005 in. intervals up to 1 in., and this enables accurate longitudinal movements of the fibre to be made. By the use of an adjustable stop accurate movements may be made along the whole length of fibres up to six inches long. Cross-sectional area variations and ellipticity along the whole fibre length can thus be measured. Further, the mechanism enables the fibres to be extended by a known amount so that dimensional changes may be studied. The instrument has been designed to be used with a projection microscope specially made for wool fibre diameter measurements.

This note discusses the problem of earthing an oscilloscope, for safety reasons, without causing earth loop interference. A special saturating choke, wound on a toroidal mumetal core, is suggested as a means of earthing which would overcome this difficulty.

A description of an ion bombardment unit is given. Ions of energies between 3 and 15 kev are supplied by a high frequency ion source with post-acceleration. The object chamber can accommodate up to twelve specimens at a time, and a low power microscope is provided for the observation of the specimen changes during the bombardment.