In the last 30 days

• Open/close The Electron Trap Mechanism of Luminescence in Sulphide and Silicate Phosphors

  G F J Garlick and A F Gibson 1948 Proc. Phys. Soc. 60 574 Tag this article

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  Phosphorescence and thermoluminescence emission from photoconducting impurity activated phosphors have been satisfactorily explained by the storage of electrons, freed from luminescence centres or other atoms of the solid, in metastable energy levels known as electron traps. Electrons escaping from these traps give rise to emission when they recombine with luminescence centres but there is a probability that they may be retrapped in empty electron traps before their final recombination with centres. The present theoretical and experimental studies attempt to determine the extent to which retrapping does occur and what effects it will have in modifying the phosphorescence and thermoluminescence characteristics. Theoretical treatment shows that there are marked differences in these characteristics for conditions when the retrapping process is present and for those when it is negligible. Experimental investigations of the characteristics of specimens of zinc sulphide, zinc silicate and strontium silicate phosphors indicate that, except under special conditions, retrapping of electrons is negligible. These results together with other work can be explained theoretically if it is assumed that electron traps operative in the luminescence process are spatially associated with the immediate neighbourhood of the luminescence centres formed by activating impurities. This new concept is also supported by the relations found between the luminescence characteristics and the dielectric changes in phosphors of the zinc sulphide type.

• Open/close The size of a dislocation

  R Peierls 1940 Proc. Phys. Soc. 52 34 Tag this article

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  Calculations are made of the size of a dislocation and of the critical shear stress for its motion.

• Open/close Cohesion

  J E Lennard-Jones 1931 Proc. Phys. Soc. 43 461 Tag this article

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• Open/close An experimental investigation of extrapolation methods in the derivation of accurate unit-cell dimensions of crystals

  J B Nelson and D P Riley 1945 Proc. Phys. Soc. 57 160 Tag this article

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  Measurements on X-ray photographs of cylindrical specimens of different absorption and thickness taken in a camera without eccentricity show that the absorption error in the apparent unit-cell dimension a is proportional to cos ²θ/sinθ + cos ²θ/θ. The plot of a against ½(cos ²θ/sinθ + cos ²θ/θ) is linear down to θ = 30° for all four specimens used. The extrapolated values for a are in good agreement, and this extrapolation function is accordingly recommended in the case of data from well constructed cameras. Other extrapolation functions are also considered, and the effect of various sources of error discussed. A table of ½(cos ²θ/sinθ + cos ²θ/θ) is given.

• Open/close The break-up of liquid jets

  A C Merrington and E G Richardson 1947 Proc. Phys. Soc. 59 1 Tag this article

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  The mean drop size produced when a jet of liquid issuing from a nozzle into the atmosphere, breaks up, has been investigated and experiments have been carried out both with fixed nozzles and with moving nozzles (attached to an aircraft) discharging backwards. A relation expressing drop size in terms of viscosity and the relative velocity between the jet and the surrounding air, applicable to both cases, is given.

  At low velocities this relation ceases to apply and drop size reaches a limiting value. Factors influencing this limiting figure are discussed and information has also been obtained on the stability of large drops.

  Measurements made on the length of the jet confirm the work of Tyler and Richardson but also disclose a hitherto unsuspected anomaly in the case of very viscous liquids.

  Similar results were obtained when a jet issued into another liquid with which it did not mix, but showed the effect of the relative viscosity of the liquids on the break-up.

• Open/close The thermal expansion of aluminium from 0° to 650° C

  A J C Wilson 1941 Proc. Phys. Soc. 53 235 Tag this article

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  The lattice spacing of aluminium has been measured as a function of temperature by means of a high-temperature Debye-Scherrer X-ray camera. The coefficient of expansion has been deduced. The results are:

  Temp.	0°	100°	200°	300°	400°	500°	600°	650° C.
  d	4.0391	4.0486	4.0592	4.0701	4.0820	4.0947	4.1087	4.1162
  10 6 α	22.0	25.4	26.5	27.8	29.9	32.5	35.5	37.2

  The systematic and random errors in d are each about 0.0001 A., the error in α from 1 to 2 per cent.

• Open/close The determination of the viscosity of liquid gallium over an extended nrange of temperature

  K E Spells 1936 Proc. Phys. Soc. 48 299 Tag this article

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  On theoretical grounds measurements of the viscosities of liquids over as wide a temperature-range as possible are desired. Gallium is particularly suited for this purpose, and the viscosity of theolten metal has been measured between its melting point (approximately 30°C.) and 1100°C. This is a much wider range of temperature than has hitherto been recorded in viscosity-measurements. It has been necessary to devise a special technique on account of practical difficulties connected with surface films. At 1100°C. the viscosity decreases by only 2 per cent per 100°.

• Open/close The thermal expansion of graphite from 15°c. to 800°c.: part I. Experimental

  J B Nelson and D P Riley 1945 Proc. Phys. Soc. 57 477 Tag this article

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  The variation with temperature of the a and c unti-cell dimensions of hexagonal Ceylon graphite has been measured over the temperature range 15°-800°c. by the X-ray powder method. At 14°·6c., a=2.4562±0.0001 kx. c=6.6943±0.0007 kx. The carbon-carbon bond length, C-C=1.4210±0.0001a.

  The a dimension shows a slight contraction up to about 400°c., a small expansion occurring above this temperature. The thermal expansion in the c direction is large; the average value for a over the temperature range is 28.3 × 10 ^-6. The complex nature of the expansion in both directions is discussed qualitatively.

• Open/close Note on the whispering gallery of St Paul's Cathedral, London

  A E Bate 1938 Proc. Phys. Soc. 50 293 Tag this article

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  The apparent disagreement between Rayleigh's deduction and those of Raman concerning the whispering-gallery phenomenon is shown to depend on the fact that Raman used a sustained source which gave rise to interference, whereas Rayleigh dealt with a whisper projected in one direction. Further, it is shown that, whereas Rayleigh attributed the success with a whisper to the fact that it is composed of high frequencies, low frequencies are projected equally well, and that actually the success attained with a whisper is due to its low intensity, which does not give rise to audible echoes. Explanations are offered of the circumferential and radial alternations of sound mentioned by Raman and Sutherland.

• Open/close The lattice-dimensions of zinc oxide

  C W Bunn 1935 Proc. Phys. Soc. 47 835 Tag this article

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  The lattice-dimensions of pure zinc oxide condensed from the smoke have been determined to a high degree of accuracy by means of X-ray powder photographs. Bradley and Jay's lattice-dimensions of quartz and Siegbahn's X-ray wave-lengths being accepted as standards, the lattice-dimensions of zinc oxide are found to be: a ₀ = 3.2426 ₅ ± 0.0001, c ₀ = 5.1948 ± 0.0003, Axial ratio c ₀ a ₀ = 1.6020 ₀ ± 0.0001, all at 18° C.

  Both the lattice-dimensions themselves and the axial ratio are lower than Finch and Wilman's electro-diffraction values which are based on the X-ray lattice-dimension of gold, by amounts larger than the combined errors of both methods. There are thus real discrepancies, which are of the order of ½ to 1 per cent.

  In view of these discrepancies, Finch and Wilman's assumption that the apparent lattice-dimension of gold is the same for 40-60 kV. electrons as for X-rays may not be correct. The discrepancies therefore represent the combined possible discrepancies shown by gold and zinc oxide.

  This does not apply to the axial ratio, which is independent of any reference substance. The difference between the axial ratios determined by electrons and by X-rays appears to point to a real difference in axial ratio between the surface and the interior of zinc oxide crystals.