Table of contents

The new devices, instruments and materials exhibited are briefly described, and mention is made of those for which significant advances in performance are claimed.

The attractive force of an ophthalmic magnet is shown to depend on the input power to the magnet and is independent of the magnet size. For a particular value of attractive force, the size depends on the safe excitation time of the magnet, that is, the time for the winding temperature to reach its safe limiting value.

By designing the magnetic circuit to optimize the attractive force, and by matching more closely than is usual in eye magnets the safe excitation time with the time required to perform an eye operation, it proved possible to obtain with a 13 1b 4 kW hand magnet, an attractive force several times greater than most ophthalmic magnets in present use. The magnet is rated conservatively at 19 sec continuous excitation. Since an operation requires only a second or so, there seems no good reason against a further reduction in excitation time to permit a higher attractive force or an even smaller size.

This instrument is designed for measuring the amplitude of voltage pulses with particular application in nuclear counting. An output pulse is obtained only when the input pulse has an amplitude lying between two preset limits; the lower limit being variable from 5 to 50 V, and the interval between the limits being 0.5, 1, 2, 3, 5 or 7.5 V, selected by a switch. The instrument can also be used as a simple discriminator. A feature of this instrument is that without readjustment it can deal with a wide variety of input pulses, ranging from 0.2 μsec to over 1000 μsec in length. In all cases the dead time is only a few tenths of a microsecond. The design, which is unusually simple, includes a high-speed threshold amplifier, an improved high-speed discriminator circuit and a new veto circuit.

The paper describes the construction and performance of a displacement gauge designed to withstand severe dynamic loading. Trials have been carried out using this gauge and satisfactory results obtained. The gauge is a variable capacitance type which gives a linear change of capacitance with displacement and this change of capacitance is used to modulate the frequency of a 2 Mc/s carrier. This modulated carrier is then passed through a discriminator circuit and amplified to operate a cathode-ray oscilloscope from which photographic recordings are made. The range of the gauge described in this paper covers a movement of 3 in. in the positive phase and 1 in. in the negative phase. The gauge gives a change of capacitance of 11.0 μμF per in. of linear movement and the accuracy is of the order of ±2%.

Apparatus is described for the measurement of the natural longitudinal resonant frequencies of metal bar specimens. The specimen is driven by the application of an alternating electrostatic force between one end and a small fixed plate parallel to it. The resonance is detected by a frequency-modulation system, working at 60 Mc/s, using the same plate as for the drive. The specimen mounting and drive arrangements are described and details given of the frequency-modulation and detection circuits.

The equipment has a considerably wider frequency range than previous designs and measurements can be made, not only of the fundamental resonant frequency, but also of the frequencies of a series of higher modes of vibration, on specimens of widely differing sizes. These observations, when corrected for small degrees of anisotropy, enable the values of the true isotropic moduli to be derived to an accuracy of one or two parts in 10³.

A means of sealing various optical materials to copper or glass apparatus has been devised. The seals have been tested under high vacuum between room temperature and 20°K, and should be satisfactory at still lower temperatures.

A cloud-chamber reprojection system is described which has been used in neutron scattering experiments with a point source of neutrons 20 in. away from the chamber. Scattering angles and ranges of recoil particles are measured directly.

An apparatus for producing a point source of X-rays, less than 1 μ in diameter, is described. A tungsten hairpin filament is employed, and two magnetic electron lenses form a demagnified image of it on a metal foil which serves as both target and window. The tube is demountable; a variety of different targets has been employed (copper, silver, tungsten) for producing X-rays of different characteristic wavelength. The maximum voltage applied has been 20 kV, but simple modifications would permit 30 to 40 kV to be used. The optimum conditions of operation are briefly discussed. The tube has been constructed primarily for the projection of X-ray shadow images of small specimens.