Table of contents

Methods available for interference microscopy of opaque objects at high wedge angles are discussed and formulae for interpreting interferograms are derived. Fringes can be resolved with wedge angles up to 30° both with the Zeiss-Linnik interference microscope and with an ordinary bench microscope using an oil immersion lens and parallel illumination (Fizeau system). The accuracy with which these angles can be measured is poor, about 10% for the highest angle resolvable, but the accuracy increases proportionately with decrease of angle. Some interferograms illustrating the capabilities and drawbacks of the methods are given.

The lattice parameters of the isomorphous compounds MnAs, MnSb and MnBi have been measured over a temperature range which includes the temperatures at which certain magnetic transitions occur. Characteristic changes develop near the magnetic transition temperatures and are of two kinds: (a) discontinuous changes of lattice dimensions (MnAs at 40°c, MnBi at 320°c), which are associated with magnetic transitions of the first order, (b) discontinuous changes of the temperature derivatives of the lattice parameters (MnAs at 130°c, MnSb at 320°c), associated with second-order magnetic transitions.

Correlation of the results on MnAs and MnSb with those obtained by Greenwald on MnTe indicates that the magnetic exchange energy for manganese atoms at a distance r apart is a maximum for r = 2.94 °.

The magnetic change which takes place in MnBi near 440°c is caused by decomposition of the material and is not a Curie point phenomenon.

The paper gives an account of an x-ray study of the copper-palladium alloy Cu₃Pd, with special reference to the variation of the degree of order with temperature as revealed by the degree of tetragonality of the structure at different temperatures. From the relation between the axial ratio of the structure and the temperature it is concluded that the transformation from the cubic face-centred structure to the tetragonal face-centred structure takes place at 460°c and the material is fully ordered at temperatures below about 270°c when the axial ratio c/a is 0.9851 and the a-parameter 3.7021 kx at 18°c. On the assumption that the degree of tetragonality gives a measure of the degree of order it is concluded that the relation between the axial ratio and the degree of order is given by c/a=1 - 0.015 W when W is the degree of order.

The paper contains a table of Chapman's function Ch(x,χ)=x sin χ ∫₀^χ exp (x-x sin χ/sin λ) cosec² λdλ to three decimal places for x=50 (50) 500 (100) 1000 and χ=20° (1°) 100° excluding values of Ch(x, χ) greater than 100.

A new method for observing the points of real contact between two solid surfaces is described. A metallized glass surface is pressed into contact with the specimen, the glass surface being deformed locally at the points of contact. The opposite side of the metallized surface is examined through the glass by phase contrast microscopy, the deformations showing bright on a dark background. Deformations of a few ångstroms in depth are detectable if their lateral extension is above the resolving limit of the microscope. Nominally flat, ground, polished, and lapped specimens of silver steel have been examined. With each specimen true contact was made within regions the sizes and distribution of which were determined by the general undulations of the surface and by the magnitude of the applied load. The differences lay in the distribution of the local contact areas within the general regions. With the polished specimen true contact was made over almost the whole of the general region but with the ground specimen it was confined to a few isolated areas along the grinding ridges. True contact with the lapped specimens was made at a very great number of areas each a few microns in linear dimension.

Some methods of reducing that noise peculiar to choked jets are investigated, the mode of operation being a reduction of one or more of the factors occurring in the gain criterion qη_sη_tη_d gt-or-equal, slanted 1, in the notation of an earlier paper. The methods used consist of the notching of the exit edge of the jet nozzle, the introduction of cambered radial vanes and ventilation of the nozzle by means of a gauze extension. The magnitude of the noise of a jet emanating from a plain exit is shown to be susceptible to the state of the exit edge and the reflecting area presented by the nozzle body in the plane of the exit. It is suggested that the presence of initial turbulence or noise will also tend to reduce the magnitude of the sound, and this effect may be of importance in the practical case of the jets of propulsive mechanisms. The methods used influence other types of noise always present to some degree, a measured reduction in actual subsonic noise only being found in the case of the radial vanes.

The (I, V) characteristic of germanium diodes is shown to depend markedly upon the wave forms used to plot it by oscilloscopic means. When the time variation of current is studied by the use of microsecond pulses a breakdown first observed by Billig occurs. The critical barrier temperature at which this breakdown occurs and the barrier radius are obtained by postulating a simple model for the contact. The possibility of the breakdown being due to thermal instability is discussed.

A method is shown for the determination of the relaxation spectrum from the data furnished by dynamical experiments, and in particular from the damping (imaginary part of the complex modulus) as a function of frequency. The circular functions are eigenfunctions of the relevant integral equation, which can therefore be solved numerically by expansion. The spectrum is obtained by first making a Fourier analysis, multiplying every coefficient by the associated eigenvalue, and then synthesizing the new Fourier series. An estimate is given of the resolving power of the method, which is limited only by the error in the experimental data. As an illustration, the spectrum of polyisobutylene in the frequency domain corresponding to the rubber-glass transition is found from published data.

A ruled grating vacuum spectrometer has been constructed in which the radiation is detected and its intensity measured by means of a Geiger counter. The working range of the instrument is about 20-200 Å. An outline is given of the principles of its design. of the procedure of adjustment, and of the method of finding the instrumental line-width.

The operation of the instrument is illustrated by experiments on the shape of the M line of molybdenum and on the N `doublet' of tungsten, in which a new structure has been detected, and which has been analysed.