Table of contents

This paper describes a tensile testing machine for microscopic specimens. It can apply loads in the range from 1 mgf to 400 gf to specimens having cross-sectional areas down to 10^-7 mm². Specimen extensions from a few angström units to 15 mm can be measured and, at constant temperature, random errors in the extension measurement system are less than 5 Å (5 × 10^-7 mm). The machine employs a torsion balance to apply the loads and a mirror-autocollimating telescope system to detect the extensions. Since it depends wholly on mechanical and optical principles it is practically drift-free at constant temperature. It is adaptable for tests in special atmospheres, for autographic recording and for tensile testing of thin films.

The basis of verification of metal Sikes hydrometers has been refined as the outcome of discussions between the Laboratory of the Government Chemist and the National Physical Laboratory. The refinement takes the form of the adoption of conventional reference conditions for the wetness of the hydrometer stem above the liquid surface and the surface tension pull acting on it. Corresponding refinement in the reference conditions associated with the glass hydrometers has been adopted by the N.P.L. These refinements will lead to greater consistency in verification and will not affect the ordinary calibration of Sikes hydrometers, except for glass hydrometers used in very dilute solutions. Full details for the precise verification of both metal and glass Sikes hydrometers are given.

Details are given of a time-resolved spectrograph with writing speeds up to 5 mm/μs (at f/12) which can be used to view weak light sources. The main limitation is that the light source must be able to be triggered reproducibly. This is necessary as the increased effective light intensity needed to obtain a usable image density in the camera of the spectrograph is produced by the accurate superposition of the light of many discharges.

An experiment is described which shows that the superposition of fifty successive discharges has been achieved with an overall variation of less than 0.3 μs.

A pulse-sampling voltmeter designed for measuring low-frequency transistor current gain under large signal conditions is described. The method consists of sampling a waveform at approximately 100 c/s with a 20 μs pulse, which gates the voltage under test at any required point of the cycle. The segment selected by the pulse is amplified and rectified, its amplitude being determined by a comparison technique in which a variable d.c. voltage is fed to the test set in place of the waveform. The equipment presents an input resistance of at least 200 kΩ on all ranges and measures voltages from 100 mv to 75 v with an accuracy to 1%; the upper limit could be extended.

This instrument was designed to save the time spent in sharpening microtome knives by hand. It sharpens by simultaneous backwards and forwards and side to side movements of the knife over an abrasive plate. The movement is derived from an electric motor. The whole instrument is portable and requires no attention except to turn the knife over periodically. An adjustable steel toe allows the facet angle of the knife edge to be correctly set. This disposes of the dangerous operation of fitting to the knife the split cylindrical backing piece supplied by the knife-makers for this purpose. In order that the knife shall not be subjected to the stresses of clamping it is held in position on the abrasive surface by cylindrical permanent magnets, which also act in keeping the knife edge in contact with the abrasive surface with an even constant pressure. The use of magnets also facilitates the placing of the knife in its position for sharpening and its removal therefrom. It is shown that the repeated examination of the edge of the knife by microscope during sharpening is necessary.

A simple mechanical trace reader is described that enables deflections recorded on paper or film to be read off, on a dial gauge, in any desired units when a movable cursor is set to the required points on the trace. Its use is illustrated for evaluating recorded temperatures, at various points and times, in a thermal model for the study of heat flow in the strata surrounding a mine airway. The trace reader gives a direct indication of the temperature difference ratio (theta_r - theta₀)/(theta_v - theta₀), where theta_r is the temperature at the point in question and theta₀ and theta_v are fixed temperatures. It can be used in many other applications, provided that there is a linear relationship between the recorded quantity and trace deflection.

A screw-driven micro-tensile tester for whiskers is described. A synchronous motor moves one of the grips at a constant velocity, whereas the second grip is connected to a load cell in which a differential transformer is employed.

A circuit is described which produces rectangular quenching pulses of an amplitude up to 2000 v, variable from zero to the maximum value. The change in voltage across the counter is independent of the counting rate. An amplifier of high gain can be used so that the sensitivity is 0.3 mV. Self-oscillation, due to the voltage rise at the end of the quenching period, cannot appear.