Table of contents

The nucleation of Gunn-effect domains can be controlled by changes in external impedance if the value of the nl product is only slightly higher than 10¹²cm⁻². This phenomenon is studied theoretically with the help of Nyquist's criterion. Experimental results with Gunn-effect diodes are reported which support the analysis.

Experiments are reported on a converter in which radiation generated in an electroluminescent cell excites a photoluminescent material, so that by suitable choice of phosphor a long decay time may be achieved. Two kinds of converter have been considered, one with the electroluminescent and photoluminescent particles mixed, and one with them separated. The dependence of transfer characteristics and rise and decay times on the construction of the panels are discussed.

The diffusion of individual atoms of rhenium on (211), (321) and (110) tungsten surfaces has been studied by field-ion microscopy. The diffusion coefficients (in m²s⁻¹) of 1 × 10⁻⁶ exp ( - 20 300/RT) for the (211), 5 × 10⁻⁸ exp ( - 20 400/RT) for the (321) and 3 × 10⁻⁶ exp ( - 23 900/RT) for the (110) surfaces were not significantly changed when diffusion occurred in an applied electric field of 0·9 MV m⁻¹ normal to the surface.

Single crystals of barite (BaSO₄) are cleaved along the (001) direction and are etched in H₂SO₄ at a temperature of 100°C. The etch pattern thus obtained consists of a random distribution of micropits and of individual isolated pits. The crystals are heated to a temperature of 500°C for two days and then cleaved and etched. It is observed that the micropits are completely absent from the etch pattern on these cleavages. The etch patterns on matched pairs, one heated before etching and the other not heated, are compared. It is conjectured that the imperfections giving rise to the crowded pits move away from their sites when the crystal cleavages are heated and collect at the sites of dislocations. The implications are discussed.

The x-ray line broadening produced in lithium fluoride powder by grinding has been analysed by a number of methods and found to be mainly due to microstrains. There was also significant crystallite size broadening, which was missed by previous workers; this was probably due to the method of analysis that was employed. Grinding cylinders and balls of varying densities were used, and it was found that as the density of the grinding cylinder and balls was increased there was an increase in the microstrain concentration and a decrease in the effective crystallite size. There appeared to be a limit to the amount of crystallite size reduction that could be produced by grinding.

Assuming the Brillouin zone to be spherical an expression for temperature diffuse scattering (TDS) for hexagonal powders has been derived. The expression has been based on the general equation for thermal diffuse scattering due to Born and its extension due to Begbie. For magnesium, where the Brillouin zone is approximately spherical, TDS line profiles have been calculated for 101 bar above0 and 0002 reflections. It has been found that for these reflections TDS peaks appear at the same positions as the Bragg peaks and the TDS contribution to the tails of both the line profiles is quite appreciable.

The magnetostatic interactions in assemblies of single-domain particles are discussed and it is concluded that large-scale interaction domain structures occur as a result of transient lateral interaction fields during particle switching. Domain structures in a number of Alnico-type alloys are reported and the effects of a.c. demagnetization and rotating field treatments examined. Remanence curves, anhysteretic remanence curves, and the angular variation of coercive force and switching field were measured for Alnico alloys and elongated single-domain materials. The main conclusions are: (i) no direct correlation can be made between domain structure and coercivity; (ii) the structures are formed primarily as a result of magnetostatic interaction; (iii) both magnetostatic and exchange interactions are important in certain alloys; (iv) a uniformly demagnetized structure of alternate oppositely magnetized particles is formed in certain alloys, and in elongated single-domain materials, after a.c. demagnetization; (v) a fanning or buckling mode of reversal occurs in both alloys and elongated single-domain materials; (vi) the form of the anhysteretic curves are governed by negative interactions within the assembly which are stronger in the high-coercivity alloys and elongated single-domain materials than in the low-coercivity alloys.

The Kerr magneto-optic effect has been used to observe domain structures on rings of 65% nickel-35% iron after annealing just below the Curie point with and without an external field being present. In general the observations are consistent with the Néel-Taniguchi theory of pair orientation, but measurements of wall movement in the constant-permeability region of the hysteresis loop of the baked alloy do not agree with predictions of theory.

Using the shock-wave technique, measurements of the radial distribution of gas temperature of free-burning alternating current arc columns have been determined. Temperature distributions have been determined at different times in the current cycle for arcs of 8 to 25 A r.m.s.

The results show that at any point in the arc column the temperature has a mean value on which is superimposed an oscillatory component having a phase and amplitude dependent upon the spatial position. The alternating current arc exhibits, therefore, a cyclic heat-wave phenomenon. From the variations of phase and amplitude of temperature oscillation, thermal diffusivity values have been deduced.

The results show that the a.c. arc has a lower temperature, lower conductance and greater power dissipation than the comparable d.c. arc. Consideration indicates that these effects result from the cyclic radial gas flow generated by the oscillatory nature of the discharge.

It is shown that four equations (covering respectively the electron energy balance, ion balance, current continuity and energy balance) are necessary and sufficient to describe the constricted positive columns of certain arc discharges in alkali-metal-seeded rare gases. The internal mechanisms and basic structural features of one such column are discussed in terms of the theory, which is shown to give a satisfactory explanation of the column properties.

The electron emission from the cathode of a high-voltage low-pressure glow discharge, under various conditions, was studied with the aid of a simple electron emission microscope. The experiments were made in a roughing-pump atmosphere at a pressure of the order of 0·1 torr. Particular attention was paid to the influence of cathode temperature in the range 20-300°C. Organic contaminants appeared to spread over the cathode by an ion bombardment assisted process of surface migration at low temperatures, but decomposed to leave an immobile residue above about 150°C. The surface could be `cleaned' by ion bombardment at an elevated temperature (250°C), and then appeared to remain in a relatively clean state while hot. Direct contamination from the gaseous atmosphere was not detected. This interpretation is discussed in regard to the glow-to-arc transition, to vapour-deposition processes and to vacuum breakdown.

The Rogowski and Bruce electrode profiles were designed for use in free-field conditions. A new electrode profile is presented which has been designed to give a uniform field over as large a gap as possible while being used inside a discharge chamber. The departures from uniformity of the electric field have been calculated, and the effects of using gap separations both greater and smaller than the design gap were investigated. Finally, the influence of the chamber wall was investigated, and found to have a negligible effect on the field near the axis. An electrode has been made to this design, and was found to perform satisfactorily.

An improved method of calculating the calibration curve for the constanttemperature hot-wire anemometer operating with high wire temperatures is described. Experimental results at high pressure and gas temperature conditions are also included.

The mobility of ions in liquid hexane has been determined by a variety of methods at fields between 0·4 and 7 kv cm⁻¹ using both air-saturated and outgassed liquid. X-rays or β particles were used to produce ion injection. At room temperature and with a drift field of 1 kv cm⁻¹ mobilities ranging between 1·2 × 10⁻⁴ and 2·4 × 10⁻³ cm² v⁻¹ s⁻¹ were found, depending on the method of measurement and the geometry of the cell. There appeared to be little difference between the behaviour of positive and negative ions and dissolved air had no effect.

The variation in apparent mobility is attributed to liquid motion which is induced by and augments the ion velocity, and depends on the hydrodynamic properties of the test cell. Experiments demonstrate clearly the importance of this effect and suggest that the true ion mobility is very small. In a low-temperature cell the mobility of positive ions was found to decrease from 1·9 × 10⁻⁴ cm² v⁻¹ s⁻¹ at 20°C to about 4·5 × 10⁻⁵ cm² v⁻¹ s⁻¹ at -50°C with an activation energy of 8·7 × 10⁻² ev. This value is close to the activation energy reported from viscosity studies and, contrary to earlier views, suggests that Walden's rule is obeyed provided that liquid motion is eliminated.

The improved Jain and Krishnan method is used to measure the thermal conductivity of cobalt in the temperature range 1200-1500°K. The thermal conductivity decreases up to the Curie point (similar 1390°K) and, after showing a minimum at this temperature, starts increasing slowly. The values of electrical resistivity and total and spectral emittances are also reported in the same temperature range. Below the Curie temperature electrical resistivity and thermal conductivity are strongly dependent on temperature. The lattice conductivity is found to be negative even after using the modified form of Lorenz ratio suggested by Backlund. Further theoretical work is necessary to understand the mechanism of conduction in cobalt.

The thermal conductivity of high-purity platinum measured by the improved Jain and Krishnan method in the temperature range 1200-1700°K is found to increase with temperature. The values of the thermal conductivity are substantially higher than the earlier measurements of Krishnan and Jain and lie within the range of values observed by other workers, confirming that the method is capable of giving accurate thermal conductivity data at high temperatures. The thermal conductivity is found to depend upon the concentration of background impurities in the platinum. The lattice thermal conductivity of platinum is found to be about 1% at 1200°K and about 15% at 1700°K of the total thermal conductivity for the purest specimen investigated. The spectral emittance at λ=0·65 μm decreases with temperature, while the electrical resistivity and total emissivity both increase with temperature.

During static electrification, when space-charge clouds are produced in substances having a very low conductivity (for example in dusts or dielectric liquids), the conductivity may increase or decrease by addition or removal of ions whether the process is one of charging or of discharging. This paper considers theoretically the discharging of space-charge clouds as controlled by the field strength and the conductivity, both of which are regarded as functions of position and time. It is shown that the cloud expands or contracts depending upon the initial value of the ratio of the conductivity inside and outside the cloud. A generalized relaxation law is obtained which is applicable if a charged cloud touches conducting walls and which thus embraces the effect of the variation with time of the conductivity of dielectric liquids.

The Maxwell Garnett theory has been extended by considering different possible values for the refractive index of the film interspaces. The effects of this on the values of n_e and k_e for island films are investigated and comparison is made with the experimental data of Philip and Trompette.

Stress-strain measurements were made on single crystals of Cu-Al alloy and the dependence of the activation volume on temperature and concentration was determined. Using the theory given in a recent paper by Feltham it was shown that there is excellent agreement between theory and experimental results.

Simultaneous measurements of the electron density and temperature distributions in a theta-pinch plasma have been made. The technique is based on the Thomson scattering of laser light and makes use of a spectrometer originally designed to measure absolute electron temperatures. Data are obtained by scanning the instrument over the plasma column and taking measurements of the scattered light and the plasma bremsstrahlung. Density and temperature distributions in plasma columns of 1 cm radius, 10¹⁶ electrons/cm³ density and 100 ev temperature have been resolved with an accuracy to 15%.

This note investigates the conditions which must be satisfied in order that the energy distribution function of electrons in a collisionless plasma may be obtained by the Druyvesteyn method using an asymmetrical double-probe system instead of a single Langmuir probe. If the error in the determination is to be less than 3% it is shown, in the case of nitrogen ions, that n₂A₂/n₁A₁ must exceed 6 × 10³, where A₁,A₂ are the areas of the main probe and reference probe respectively and n₁, n₂ are the electron densities in the vicinity of the two probes.

Mechanisms which may act as secondary processes in the streamer breakdown of isotropic gases and air are considered. In the case of air it is shown that mechanisms connected with the collisions of excited oxygen molecules with ground-state nitrogen and oxygen molecules play the primary role.

It is shown from some preliminary experimental investigations that it is possible to use the appearance of acoustic subharmonic waves as a means of detecting the onset of mechanically induced cavitation.

A method is described which permits exact tuning of a nuclear maser oscillator type of magnetometer using a liquid or gaseous maser medium which flows through two decoupled coils in succession.

Metal crystals which have been faceted by suitable heat treatment in an external furnace show a clear low-energy electron diffraction pattern from the low-index facets. Negligible background intensity is seen due to elastic backscattering from the non-crystallographic continuation surface between the facets, even when the facets occupy less than 20% of the specimen area. Sharpness of the facet spots indicates good crystallographic perfection of the facet surfaces.