Measurements of flux penetration and dissipation at 50 c/s have been made on niobium samples shaped to approach closely the case of slab geometry with a tangential applied field. When corrected for end effects the measured quantities are independent of sample thickness, which varies from 0·125 to 0·004 in. Comparison of the experimental results with the Bean-London model yields little quantitative agreement.
A Hall effect measurement technique is described which overcomes some of the difficulties presented by materials combining a small Hall angle with a large temperature coefficient of conductivity. The major problem of `offset' voltage suppression is dealt with by using a direct primary current in conjunction with an alternating magnetic field and narrow-bandwidth phase-conscious signal detection.
The coating of filaments of glass or of other materials by passing them through a molten metal globule is described. An analysis of the process of freezing by the radial conduction of heat into the fibre is given and good agreement found between predicted and measured values of the coating thickness. `Blobs', large lumps of metal adhering to the fibre, are shown to be caused by the presence of the oxide film surrounding the metal globule and can be eliminated by using an inert gas blanket. There is a discussion of factors affecting the strength of aluminium-coated silica fibres, showing that diameter variation plays only a small part in determining the variance of strength. Chemical interaction at the interface accounts for the largest part.
In this letter the author extends an earlier treatment of image formation in a stratified medium to the case when the refractive index μ varies with height y as μ = (y/yk + b)1/2. It is seen that this leads to the formation of two virtual images of an object.
It is shown that the relative size of the light source in Wootten's apparatus does not affect the interpretation of the results.
The heat capacity of potassium cobalt trifluoride was recorded over the range 80-300°K. A peak at 109·5°K is attributed to an antiferromagnetic-paramagnetic transition. The values of heat capacity, entropy, enthalpy and free energy at 298·15°K are 28·84 cal mole-1 degK-1, 39·40 cal mole-1 degK-1, 5669 cal mole-1 and - 20·26 cal mole-1 degK-1, respectively.
The equations that describe the behaviour of a time-unsteady plasma column have been simplified by neglecting energy transfer by radiation and convection. Furthermore, the thermal diffusivity of the plasma was assumed to be constant and a simplified expression for the electrical conductivity was adopted. Thus the energy equation was cast into a universal form, requiring the specification of only two parameters to obtain solutions. Although some approximate closed form solutions were obtained it was necessary to solve the general a.c. arc problem on a digital computer. In view of the good agreement between the theoretically derived voltage waveforms and those found from a specially designed a.c. cascade it is concluded that the behaviour of the low current a.c. arc can be described by the solution of a complicated heat conduction problem. These solutions have the advantage over older theories that they can be directly related to measurable or calculable quantities that characterize a particular gas.
Various types of domain structure are described and their relationship to growth conditions recorded. Possible causes of the effects observed are discussed.
A relatively detailed knowledge of the stress distribution around the fibres in a composite material is required in order to predict the mode of failure. Idealized models of a reinforced material, consisting of an isolated fibre embedded in a brittle matrix, have been analysed by the photoelastic technique. Complete stress distributions have been obtained in the vicinity of both square- and semicircular-ended fibres with the composite subjected to uniaxial tension. It is interesting to find that the maximum tensile and shear stresses induced at the tip of the semicircular fibre are significantly greater than the corresponding values for the square-ended fibre. This apparent anomaly is explained by the mechanism controlling the load transfer from the matrix to the fibre.
The motion of conducting particles in electrically stressed dielectric fluids has been examined with the intention of explaining enhanced charge transfer between electrodes in the fluid-particle system. The motion of the particles is due to induction charging and the consequent forces exerted on the particles by an electric field. It is shown that the usual assumptions relating to the velocity of such particles are not substantiated by experiments. The dependence of the enhanced charge transfer on the applied field is derived for one particular case. For this case the study of particle motion shows that particles of different sizes move with a charge equivalent to a maximum voltage on the particle of the order of 16 v. If it is assumed that the particles initially become charged when in contact with one of the electrodes, and on leaving the electrodes are shielded so that the effective charge is reduced, it is possible to show that the current is proportional to the square of the applied voltage, which is in agreement with experimental results.
Determinations of primary α and secondary ω ionization coefficients in helium at pressures p0 near to atmospheric and low values of E/p0 (E is the electric field) show that α/p0 = f(E/p0) and ω/α = ϕ(E/p0) for 30 < p0d < 730 cm torr. The results obtained for ω/α and its dependence on E/p0 are shown to be adequately accounted for on the basis of the assumption that the predominant secondary ionization process in helium at high pressures is the destruction of metastable states in the gas with the consequent production of non-resonance photons which liberate secondary electrons at the cathode.
A switching action has been observed in a two-terminal sandwich structure of the form silicon-silicon-dioxide-aluminium. The application of an increasing positive bias to the aluminium electrode results, at some particular bias, in a change from a high-resistance to a low-resistance state. The reverse change of state is accomplished by the discharge of a condenser across the device.
An 8·6 mm microwave bridge has been used to determine the complex permittivity of water over the temperature range 1-60°C to within a standard error of 0·25 in both ' and ". These results indicate that the high-frequency dielectric constant [infinity] varies with temperature from 4·9 ± 0·3 at 1°C to 3·1 ± 0·8 at 60°C, which is a new proposal for water. As with previous investigations, it is difficult to discriminate between Debye behaviour and a small distribution of relaxation times, but the present measurements conform to the latter type of process below room temperature with increasing approximation to a single relaxation time as 60°C is approached. Values of the relaxation wavelength λs, with an error of ± 1-2%, are also given and are slightly lower than those published hitherto.
The theory of electron tunnelling through a thin dielectric film is modified by taking into account the penetration of the electrodes by the applied field. The numerical predictions differ only slightly from the existing calculations but some significant differences in the asymmetry of the current-voltage characteristics are produced. In particular, it is shown that the direction of easy current flow can reverse twice with increasing voltage. This depends on the barrier asymmetry and the relative amount of field penetration of the junction electrodes. Also the peak of the thermal (J, V) curve no longer occurs at a voltage equal to the metal-insulator barrier height, but at a slightly higher voltage. Some of the predictions of the modified theory are confirmed by experiment.
The statistical distribution of multiple emission of secondary electrons has been studied when He0 atoms in the energy range 1-5 kev impinge on pure aluminium in a secondary-electron scintillation detector. The results show that the distribution function is Poisson type, and by analysis of only two peaks on the pulse-height spectrum the fraction of incident He0 atoms which fail to cause the emission of an electron can be estimated. Secondary-electron coefficients γ have been obtained from the experimental distribution functions for He0 and Ar0 incident on Al. For He0, γ varied smoothly from 1·3 to 2·4 in the energy range 1-5 kev. The effect of argon ion-cleaning of the cathode has also been studied.
The effects of alpha particles in AgCl crystals which lead to tracks which can be rendered visible by `decoration' techniques have been found to have in some cases a very short lifetime which is a function of the structural properties of the crystals and the doping materials used. The formation of visible tracks therefore depends on the time interval between irradiation and decoration. This property could be made use of for the triggered recording of tracks.
A new descriptive formalism is introduced to represent the state of a bonded random network of fibres, such as paper, when subjected to mechanical deformation. Energy is considered to be distributed among the lengths of free fibre between inter-fibre bonds and a criterion for the fracture of these bonds is postulated. The theory is used in the present paper to derive the cumulative fracture of bonds as a function of time in a network which suffers a uniform rate of elongation. Good agreement was obtained with experimental recordings of the failure of bonds in a thin sheet of paper. No systematic influence of the rate of straining on the mode of bond failure was detected.
This paper describes a study of the temperature dependence of the dynamic characteristic and friction in the melting and crystallization region of linear polyethylene, isotactic polypropylene and their 9: 1, 5: 5 and 1: 9 mixtures.
Ionization and attachment coefficients, α and η, have been obtained by observing, in pure chlorine at pressures of from 1 to 10 torr, the variation of prebreakdown ionization currents with the distance between parallel electrodes at reduced fields in the range 70-150 v cm-1 torr-1. Values of α - η have been obtained for values of E/p up to 250 v cm-1 torr-1. The reduced coefficient α/p rises from 0·22 at E/p = 70 to 2·0 at E/p = 150, while η/p falls from 0·84 to approximately 0·25 in the same range. No pressure dependence could be found. These values of the attachment coefficient are much larger than those indicated by an extrapolation of the results of Bailey and Healey.
Electron drift velocities have been measured in nitrogen and hydrogen over the E/p range 4-40 v cm-1 torr-1 using a drift tube apparatus incorporating fast shutters. A brief description of the apparatus and technique is included.
