Table of contents

The characteristics of the polysiloxane chain are described, and their effects on the properties of silicones are discussed. Silicones possess excellent thermal stability over a temperature range of more than 300° C, low surface tension, good electrical properties and a high degree of water-repellency, and are effective as release agents. The mechanical properties of the condensed polysiloxanes are poorer than those of most organic polymers at moderate temperatures, but are markedly superior at extremes of temperature. Silicones are available in the form of fluids, rubbers and resins; the manufacture and properties of each are described, and their special applications are discussed.

Rhodium-plated kata thermometers were developed, as rhodium is a more stable coating than silver. The rhodium-plated kata thermometers were tested in non-turbulent air streams at the National Physical Laboratory, Teddington. Formulae for estimating true air velocities from cooling rates of the kata thermometers are given. Accompanying graphs facilitate the calculation of velocities. Cooling of the instruments is not directly related to the square root of air speed at low velocities. From zero speed (still air) up to a velocity of 0.5 m/s, plotting of cooling against velocity results in a parabola, and from 0.5-1.3 m/s in a straight line.

The design and construction of a new high-resolution spectrograph for the visible and ultraviolet regions of the spectrum are described. The optical system is that due to Ebert, in which a plane diffraction grating is used in conjunction with a spherical mirror as collimator and camera. This system has several advantages over conventional concave grating mountings; a stigmatic image is formed and wavelength change is accomplished simply by rotation of the grating. The instrument described can also be used as a spectrophotometer, and can be adapted for use in the vacuum ultra-violet.

The fringe spacing in transmission interference microscopes is shown to require a correction due to the oblique rays, with respect to the ordinary value valid for perpendicular rays. The spacing, and accordingly the measured path differences, t(n_x - n₀), are smaller than the normal values (opposite to the case of reflexion microscopes). Calculations are made for different values of condenser N.A. and varied parameters, showing corrections up to - 15%. The fringe intensities also decrease with the order of interference when it is low. Experiments confirm these findings. The effect is discussed in relation to some general ideas of (x, y)-fidelity and (z)-fidelity as obtained in a limited optical information channel.

The counter uses a cylindrical crystal of thallous chloride 12 cm long and 9.3 cm in diameter viewed by a photomultiplier tube, and is capable of giving pulses of up to 3 V in height without serious departure from linearity. The pulse height resolution for 78 MeV photons is 34%, and both the mean pulse height and resolution remain constant to within 5% over an entrance aperture 5 cm in diameter. The performance of the counter is discussed in some detail, and it appears that the resolution is limited principally by strong absorption of ultra-violet light in the crystal.

A compact resistance and voltage comparator employing an isolating potential circuit with mechanical choppers has proved to be a versatile and accurate laboratory tool. Voltages can be compared with an accuracy of 10^-8 V and resistors with an accuracy of 2 parts in 10⁶ (d.c. operation) or 2 parts in 10⁵ with automatic cancellation of thermal e.m.f. (square wave operation). When a d.c. amplifier and recorder are used, voltage and resistance differences may be recorded with an accuracy of about 2 parts in 10³.

A windowless gas target capable of maintaining a pressure of up to 2 mm of mercury has been constructed to determine the differential cross-section of charged particle reactions. Targets of this type have been used to examine (d, p) and (d, α) reactions in the range 15 keV to 1 MeV.

The resistance of the platinum-rhodium alloy varies with temperature so much that a fully automatic programming controller must incorporate a device for adjusting the voltage applied to the winding as the furnace temperature changes. The way in which this was achieved, by means of a servomechanism, is described.

An electronic apparatus is described for the determination by a resonance method of Young's modulus and the modulus of rigidity of materials in the form of cylindrical rods. Results can be obtained at strains as low as 10⁹. The method of calculation of the moduli from the observed resonant frequencies is given.

The note describes a simple modification to progressive codes used in coded-plate digitizers so that a signed readout can be obtained. Little increase in the complexity of the decoding equipment is necessary and neither the accuracy of readout nor the maximum tolerance of the digitizer to mechanical imperfections is impaired.