Showing papers by "National Physical Laboratory published in 1974"

Newton-type methods for unconstrained and linearly constrained optimization

Abstract: This paper describes two numerically stable methods for unconstrained optimization and their generalization when linear inequality constraints are added. The difference between the two methods is simply that one requires the Hessian matrix explicitly and the other does not. The methods are intimately based on the recurrence of matrix factorizations and are linked to earlier work on quasi-Newton methods and quadratic programming.

On the effects of boundary-layer growth on flow stability

Abstract: The stability of small travelling-wave disturbances in the flow over a flat plate is discussed. An iterative method is used to generate an asymptotic series solution in inverse powers of the Reynolds number Rx = Ux/v to the power one half. The neutral-stability boundaries given by the first two terms of this series are obtained and compared with experimental data. It is shown that the parallel flow approximation leads to a valid solution at very large Reynolds numbers.

Cohesion margin of copper

Abstract: Measurements on the mechanical properties of copper-bismuth alloys show that the only effect of bismuth within the solid solution range is to reduce the true tensile strength of the alloys—there is no detectable effect on plastic behaviour or on the elastic limit. Intergranular fracture occurs by the formation of grain boundary crack nuclei, these being seen in alloys containing as little as 0·002 wt.-% Bi. In addition, by measuring the effect of bismuth on surface and grain boundary energies of copper, it is shown that bismuth is present at the grain boundaries at monolayer levels, as true Gibbs segregation. It appears that the brittleness is due simply to reduced grain boundary cohesion. The measured relative cohesion of the copper-bismuth alloy is approximately half that of pure copper: such a reduction in grain boundary cohesion therefore converts ductile copper into a brittle metal, and thus defines a margin of cohesion. Reduced grain boundary cohesion favours the nucleation of cracks where ...

The effect of air and vacuum environments on fatigue crack growth rates in Ti-6Al-4V

Abstract: Fatigue crack propagation studies in vacua of 1.33 mN m-2, on Ti-6 A1-4V, at growth rates of 10-7 to 10-4 mm/cycle have shown that a threshold for growth exists at ‡K values of 6.3 to 7.6 MN m-3/2. The value of the threshold level is microstructure dependent, but growth above this value was structure insensitive according to both growth rates and fracture surface observations. Some slow (≈ 10-8 mm/cycle) crack extension was observed below the threshold values but prolonged cycling reduced the growth rate to a vanishingly small level. Fracture surface observations indicated that growth in this region was microstructure sensitive. Comparison with previously performed air work on the same material showed that while structure insensitive growth rates in vacuum were slower than those in air by a factor of 3 to 4, the low ‡K value structure sensitive rates were slower than the air ones by at least three orders of magnitude. A hypothesis is proposed to explain this in terms of a propagation mechanism for the structure sensitive mode of fatigue crack growth.

High-Resolution Submillimeter-Wave Fourier-Transform Spectrometry of Gases

Abstract: Modem interferometric techniques now permit the measurement of broad-band absorption spectra of gases at submillimetric wavelengths to high resolution with comparative ease. This paper describes briefly some new spectra of H/sub 2/O, N/sub2/O, and SO/sub 2/ in the 10-40 cm/sup -1/ (0.33 to 1.4 THz) region, at a resolution of 0.05 cm/sup-1/.

Thermal gradients in differential scanning calorimetry

Abstract: A combined static and dynamic temperature calibration is described. The static calibration corrects the instrumental dial temperature reading. The dynamic calibration has instrumental and material components and therefore varies from specimen to specimen. It is obtained from individual DSC curves and so removes uncertainties in sample temperature due to varying mass, geometry, and heating rate. The instrumental performance is improved and specific heats may be obtained to an accuracy of ±1%.

Measurement of the speed of light

Abstract: WE report here the completion of a determination of the speed of light at the National Physical Laboratory1. The value was obtained from the product of the measured frequency and the wavelength, determined through up-conversion, of the radiation from a CO2 laser stabilised to the R(12) transition of CO2 at 9.3 µm.

A comparison of doctor and computer interrogation of patients.

Abstract: Patients were interrogated for the presence of gastrointestinal symptoms by either a doctor-doctor pair or a doctor-computer pair, in each case independently. A study was made of the acceptability of computer interrogation to the patient and the accuracy of the responses elicited was measured by estimating the errors incurred by doctor and by computer using a model based on information theory. Computer interrogation was acceptable to the majority of patients but was less accurate than doctor interrogation.

Direct 825th harmonic mixing of a 1 GHz source with an HCN laser in a Josephson junction

Abstract: Signals near 1 GHz were multiplied in frequency by up to 825 times and mixed with the hydrogen cyanide laser frequency at 891 GHz in point-contact Josephson junctions. The 1 GHz signals were generated by direct multiplication from a 120 MHz quartz-crystal oscillator, and no microwave transfer oscillator was needed. The maximum IF output powers obtained in these and other harmonic mixing experiments (using the same laser) were found to agree quite well with a simple theory which predicts a limit proportional to the square of the frequency being multiplied in the Josephson junction. This provides a criterion for determining the lowest frequency at which to begin multiplication in a Josephson device.

Zeeman Splitting and Chemical Shifts for the Shallow Donor States in CdTe

Abstract: Zeeman splitting of the shallow donor states of very high purity n-type CdTe is studied by means of extrinsic photoconductivity experiments. Pine structure due to chemical shifts of a t least six different donor types is observed on the 1s 2p and the 1s 3p transitions. The magnitude of the chemical shifts observed (1 to 8 cm-l) suggests a lower value for the effective Rydberg (106 cm−1) than the value found in previous magneto-optical experiments. Resonant polaron effects are detected on the 1s 2p+ line a t high magnetic fields. Le clivage Zeeman des niveaux donneurs peu profonds dans le CdTe de type n de tres grande pureti: est etudie par des experiences de photoconductivite intrinskyue. La structure fine due aux deplacements chimiques de six types au moins de donneurs differente est observee sur les transitions 1s 2p et 1s 3p. l'amplitude des deplacements chimiques observes (1 a 8 cm−1) suggere une valeur de la constante de Rydberg effective plus faible que celle trouvee dans les precedentes experiences magneto-optiques. Des effets polarons resonnants sont detectes a champs magnetiques eleves sur la raie 1s 2p+.

The SCI/IUPAC/NPL project on surface area standards

Abstract: Values of the nitrogen BET specific surface areas calculated from data supplied by many laboratories on four currently available surface area standards are presented and compared. Analytical data, representative adsorption isotherms and typical BET plots are given. Various factors affecting the accuracy of nitrogen BET surface area measurements are summarised.

H 2 O, O 3 , N 2 O and HNO 3 in the arctic stratosphere

Abstract: MEASUREMENTS of the total amounts and average mixing ratios of a number of stratospheric trace gases at altitudes above 15 km have previously been published1–3. The measurements were derived from submillimetre wavelength emission spectra, recorded on board Concorde 002 during high altitude test flights.

In-situ observations of the annealing of liquid lead inclusions entrained in an aluminium matrix

Abstract: The changes in configuration of cylindrical inclusions of molten lead entrained in an aluminium matrix have been observedin-situ using hot-stage high voltage electron microscopy. Three distinct types of events were observed and analysed. The cylindrical inclusions spheroidized over a wide temperature range due to capillary forces; the resultant spherical inclusions migrated in the temperature gradient caused by electron beam heating; and impinging inclusions coalesced due to capillary forces.

D-region in disturbed conditions, including flares and energetic particles

Abstract: A unified picture of the chemistry of the normal and the disturbed D-region is presented. Under disturbed conditions we include: (a) solar flares, (b) PCAs and (c) solar eclipses. The connecting thread is the six ion model of Mitra and Rowe, with appropriate modifications where necessary. Several areas of identity in the observational results of flares and PCAs are reviewed. These include: the dominance of q(O2+) over q(NO+) due to X-rays in the first case and energetic particles in the second; drastic decrease in the effective loss coefficient during both flares and PCAs; disappearance of water cluster ions down to 73 km during day and 78 km at night during PCAs; the reversion of negative ions to their basic forms (O− and O2−) during PCAs. For solar eclipses, the effects are essentially a mirror image of these due to flares and PCAs.

Iodine Stabilized Laser Strainmeter

Abstract: Summary A new type of laser strainmeter has been operated in Queensbury tunnel, Yorkshire, England. It has design features which possess fundamental and practical advantages over other laser strainmeters which have been constructed. The system uses two helium-neon lasers which operate at 633 nm. One illuminates a Michelson interferometer with a 54 m long arm in which strain is to be measured. The laser frequency is controlled to keep a particular fringe at the outputs of the Michelson interferometer as the path difference in the interferometer alters. The other laser is stabilized by saturated absorption in iodine vapour, and has extremely good frequency stability over very long periods. The frequency difference between the lasers is measured to give a signal directly proportional to the strain in the interferometer with very high resolution and linearity. Twelve weeks of tidal data have been produced by the instrument.

A theoretical study of the Nd: Yb: Er glass laser

Abstract: This paper discusses the prospects of getting efficient laser generation from Er at 154 μm with Nd and Yb co-dopants in an unclad glass rod. Target outputs at room temperature are 50 mJ free-running or 30 mJ Q-switched. Ion-to-ion transfer rates are deduced from fluorescence lifetime measurements on a variety of Li-Mg-SiO2 base glasses. Good energy transfer can be obtained from Nd to Yb and from Yb to Er. Unfortunately there is also some transfer back from Er to Nd. Measurements on flash-tube spectra and flash-tube efficiency are combined with glass absorption spectra measurements and pumping cavity efficiency calculations to give values of pumping efficiency for various concentrations. By combining the efficiency results with the ion-to-ion transfer results we predict the pump energies needed to get the target output for various rod dimensions and ion concentrations. Pump pulse duration is varied continuously with the energy into the tube to keep down tube loading and give an acceptable tube life. Optimum concentrations (compromised between the various targets) are found to be 42×1020 for Nd, 13×1020 for Yb, and 03×1020 ions/cm3 for Er, and the optimum dimensions, length about 3 cm, diameter about 03 cm, giving a threshold energy of 60 J and input energies for the target outputs of 80 and 120 J. Limitations on the accuracy of these predictions are discussed. Smaller inputs are obtainable if the design is made more specific to the target output.

Accurate wavelength measurement on up-converted CO 2 laser radiation

Abstract: WAVELENGTH measurements, which are part of an accurate determination of the speed of light1, have been made on the radiation from a carbon dioxide laser, stabilised to the R(12) transition at 9.3 µm by saturated fluorescence in an external CO2 cell, which has a measured frequency2 reproducible to better than one part in 109. The problems of a direct intercomparison of infrared and visible wavelengths were avoided by mixing the CO2 radiation with light from a 10-mW He–Ne laser at 0.63 µm to give a difference frequency sideband at a wavelength of 0.68 µm. This mixing process, known as up-conversion, was performed in a cooled, single crystal of proustite3,4. The wavelength of the CO2 radiation was then determined using the relationship λ9.3 = λ0.63/(1—R), in which it is assumed that c is the same for the two visible radiations. The wavelength measurements were thus of the ratio R = λ0.63/λ0.68 and the value of λ0.63 with respect to the primary length standard.

The critical temperatures and pressures of thirty organic compounds

Abstract: Critical temperatures and pressures of some organic compounds were measured. There is good agreement between the observed critical pressures and values obtained by extrapolation of equations representing the variation of vapour pressure with temperature in the region of the normal boiling point. An incremental scheme relating critical temperatures to normal boiling points is described and is used for the correlation of the critical temperatures, obtained in this and other investigations, of more than 150 compounds.