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The Institute of Physics and The Physical Society held a conference on nuclear physics in the buildings of the Departments of Physics and Mathematical Physics of the University of Birmingham on 17th, 18th and 19th April 1961. The subject-matter discussed covered a wide field of experimental and theoretical research.

The Spring Conference of the X-ray Analysis Group of The Institute of Physics and The Physical Society was held in the Chemistry Department of Glasgow University, on 6th and 7th April 1961. Dr. C. W. Bunn, Chairman of the Group, presided at the first and third sessions, whilst Professor J. M. Robertson, F.R.S., was in the chair during the second session. The Evening Discourse on 6th April was given by Professor R. V. Jones (Aberdeen University), who discussed some aspects of the direction of the national scientific effort in connection with the war of 1939-45 under the title `Philosophers and Kings'.

A thermionic generator of electricity is essentially a diode valve in which electrons emitted from a hot cathode flow to a cooler anode, producing an electric current. From being scientific curiosities, such devices have become the subject of intense research activity in the last three years, and it is now clear that they have considerable possibilities as useful generators in a number of fields.

This article reviews this recent work and makes an assessment of the present position and future trends. Although engineering design and applications are considered, the emphasis is on the physical processes associated with these devices, and the progress which has been made into understanding them.

The heat treatment of granular carbon aggregates in various ambient gases has been studied. In inert gases (hydrogen, nitrogen, argon, etc.) and in vacuo, either a constant or a decreasing electrical resistance was observed for increasing heat treatment temperature. Ambient gases containing either free or combined oxygen produced a sharp resistance maximum at about 530° C. The increase in resistance is attributed to the formation of a non-conducting surface oxide layer which decomposes on increasing the heat treatment temperature.

The influence of conductivity on the transfer of matter at break (6 V, 4.3 A, 0.06-20 μH) was investigated for precipitation-hardened gold alloys and a palladium alloy with super-structure. For the gold alloys the degree of transfer depends strongly on conductivity in the regions of residual transfer and of normal arc, but is hardly affected in the region of the short arc.

An instrument has been designed for measuring the dielectric constant of a material in the form of a single cylindrical filament, and experiments are described in which it is used for measurements on nylon. The filament is placed centrally between two silvered optical flats, which form a parallel-plate capacitor and serve as the tuning capacitor of a radio-frequency oscillator. According to an existing theory, the change in capacity due to the presence of the dielectric is given by:

δC = ((b²H/2D²) tanh πB/2D)/(( + 1)/( - 1) - π²b²/3D²)

(where b is the radius of the filament, H the length of the capacitor, B the width and D the distance between the plates) and δC can be determined from the corresponding change in oscillator frequency, if the electrical parameters of the circuit are known. The experimental observations confirmed the general form of the equation, but they were not sufficiently accurate to verify the second-order term π²b²/3D². A value of 3.0 was found for the dielectric constant of nylon at 6 to 7 Mc/s, which is in reasonable agreement with values of 3.1 to 3.4 recorded by other workers for both filament and bulk nylon.

A theoretical treatment of the turbidity of monodisperse emulsions is discussed and its application to polydisperse systems indicated. Modifications required to commercial spectrophotometers for carrying out spectroturbidimetric studies are described. The determination of both mean globule size and concentration of the suspended phase in homogenized emulsions is illustrated.

Twenty selenium barrier layer cells, made by a modern commercial process with a variety of modifications, were examined with irradiation of x-rays in the 15-40 kev energy range and the behaviour was compared with their response to light. The x-ray sensitivity is found to be independent of intensity of irradiation at constant x-ray tube anode voltage, but linearly proportional to anode voltage at constant anode current. The form of transient response to x-irradiation depends on the intensity of irradiation and the previous history of cell irradiation, a time of rest of about 2 hours being required to achieve the `first-irradiation' response. The fatigue effect does not occur in the steady-state values to photovoltage and photocurrent. There is some evidence for a qualitative relationship between x-ray and optical photovoltage sensitivities of a cell, but there is no relationship between the photocurrent sensitivities.

Measurements have been made of the rate of deposition of sputtered molybdenum films in the abnormal glow discharge in inert gases over the pressure range 3-21 mm Hg, as a function of the gas pressure p and the current i through the discharge tube. The rate of sputtering was found to be proportional to (i/p)^2.5 in both neon and the Penning mixture 99% neon-1% argon. In helium sputtering was negligible, but the addition of only a trace of neon caused appreciable sputtering to occur.

The use of radioactive isotopes in the measurement of the transfer of metal from one electrode of an electrical contact to the other is described and details of the experimental procedure are given. The relation between matter transfer and circuit inductance at very low values of the inductance has been determined for platinum and palladium contacts operating at potential differences of the order of one volt. It is shown that for these metals there is no range of inductance down to 10^-8 H over which the transfer is independent of inductance. The amount of transfer in relation to the volume of the molten metal bridge between the electrodes is considered, and the significance of the results in the light of theories of the phenomenon of transfer is discussed.

Experimental data of the drag exerted by the walls of a cylindrical vessel on a sphere falling axially down it through a liquid are given for Reynolds numbers, based on the diameter of the sphere, between 0.05 and 20 000. Existing wall-correction formulae are examined in the light of the new data, the conclusion being that the Francis (1933) and Munroe (1888) equations are the most reliable in the laminar- and turbulent-flow regions respectively. Graphs show the correction to be applied in the intermediate-flow region.

Foils up to 2.5 × 10^-3 cm in thickness, of Cu, Ag, Pd and Au, have been evaporated on to LiF and mica crystals, epitaxial growth was observed within fixed temperature ranges for each substrate-metal combination. X-ray diffraction of the foils produced sharp patterns of spots which indicated a principal orientation of crystallites in direct alignment with the substrate crystal, together with twins whose presence was confirmed by metallography. The observations are discussed in terms of a nucleation and growth model.

A theoretical and practical study has been made to compare the optical performance of single layer λ/4 TiO₂ and double layer λ/4 MgF₂-λ/4 TiO₂ coatings when used as beam-dividing systems. Theoretical and experimental values of the spectral response for light arriving at normal incidence and at 45° incidence are given together with the values for reflectance as a function of angle of incidence for monochromatic light. The optical characteristics of glass-film-glass beam dividing systems are also investigated. It is further shown that the efficiency of a system consisting of a reflecting single λ/4 layer between glass boundaries is improved by the introduction of a second λ/4 low refractive index film layer.

An investigation has been made to see whether the secondary X-ray photons arrive in a random manner in a commercially available X-ray fluorescence spectrometer, and if so, how may the available counting time be best used. A sufficiently large number of observations has been taken to allow a full statistical analysis to be made and compared with theoretical predictions. In addition, an experimental investigation has been made into the timing accuracy and its effect on the final count rate.

It was found that for very high counting rates, above 10 000 counts/s, or long counting times, above about 2 minutes, small instrumental errors tend to occur, but for count rates generally obtained in practice, the X-ray photons are emitted in a random manner and a Poisson distribution is obtained. In practice one could expect a single observation to be within 0.6% of the true value for a 256 000 count or 1.3% for a 64 000 count.

The necessary statistics for measurements of a given accuracy are set out in a form readily usable by the X-ray spectroscopist, together with formulae and procedure for obtaining the greatest net accuracy when only a limited time is available.

The resistivity of a sample can be deduced from the change in mutual inductance between two coils when the sample is inserted. It is shown that with simple equipment for measuring mutual inductance over a range of frequencies, the method can be used to measure resistivities from 2 × 10^-9 ohm cm upwards, and the necessary functions are tabulated.

Discrete voltage pulses, of amplitude depending on the energy of the incident radiation have been observed in the photoconduction of cadmium sulphide at room temperature. Between 4800 Å and 8000 Å the pulse height varies in a similar manner to the photocurrent with wavelength. Below 3500 Å it is closely proportional to the energy of the radiation. Cadmium sulphide can be used as a spectrophotometric device in this region when only small numbers of quanta are available.

The intergranular cohesion of granular masses is investigated with an apparatus which in principle is a Couette viscometer. Rheologically, the granular mass behaves as a plastic solid, with the difference that the plastic yield point S₀ of a granular mass is a function of the pressure p which acts normally on the yielding plane. In an (S₀, p) diagram, a cohesionless mass will be represented by a straight line passing through the origin and having a limiting slope of 0.193, while a granular mass with cohesion will show a lesser slope, the departure from the limiting value being a convenient measure of the cohesion. Powders which are not free-flowing will be represented in the (S₀, p) diagram by lines, which are not necessarily straight, with an intercept on the S₀ axis.

The cohesion of a few samples of sand of different granularity and that of a few other granular masses has been measured.

Measurements have been made of the relative photoelectric quantum efficiency η(λ) of SbCs₃ cathodes of photomultipliers with Pyrex and quartz windows, from λ = 200-650 mμ. η(λ) depends on the window transmittance W(λ), the cathode absorbance A(λ, d), the absolute photoelectric quantum efficiency Q₀(λ) and the photoelectron escape probability f(λ, d). Comparison with other data has allowed the separation of these factors, yielding inter alia the absorption spectrum of SbCs₃ down to λ = 220 mμ, and the absolute photoelectric threshold curve Q₀(λ) plotted against λ. The optimum thickness for a thin cathode is d = 200 Å. Errors in other published data are noted, and methods of increasing η(λ) and of improving scintillation detectors are discussed.