Table of contents

Considers the use of a hot wire anemometer placed in a supersonic rarefied gas flow under free molecular conditions. This requires the determination of the radiative heat transfer to the gas and of the heat loss by conduction to the wire holders. A new technical improvement which consists in locating bead thermistors at the hot wire extremities has permitted the attainment of a well defined stabilised probe regime. Coupled with an electron gun, to measure the density, the probe has been tested in a free jet at Mach number M=2, and static pressure p_st=2.64 mPa (0.02 mu mHg). It has been also used to explore the boundary layer of a flat plate at Mach=2. Velocity profiles and velocity directions at each point have been deduced from the probe response.

A copper holder is described for conveniently measuring the resistance of a platinum resistance thermometer immersed in ice or other baths. The characteristics and the advantages of the holder are discussed.

A dual pilot operated valve is described which is capable of operating at very low temperatures and which is suitable for automated switching of the flow of cryogenic liquid. The valve lacks the intricacy that makes many commercial valves unsuitable for low temperature operation and has operated with high reliability for dispensing liquid nitrogen to an experimental system in which it was important that a rapid shut-off of supply was achieved when required. The valve may be opened and closed automatically by application of 300 kPa to the appropriate pilot port, and in the open condition delivers liquid nitrogen at a rate linear with driving pressure and consistent with a flow characteristics of 0.50 Pa mm^-3 s.

An inexpensive method, using a battery-operated micro-ammeter, for measuring the electrical conductivity of surfaces is described. It is specifically designed as a relatively simple and rapid means of testing antistatic polymer film.

This paper considers two sources of error in thermogravimetry which previously have received insufficient attention, namely, instability in the inclination of microbalance supporting structures and specimen temperature measurement. For the Cahn RH microbalance, for example, which is basically a beam system dynamically balanced by an electromagnetic torque, principles of mechanics are applied to a simple model to obtain the functional variation of the microbalance output in terms of weight loads, geometric parameters and the inclination of the balance supporting structure. Together with this treatment, measurement of apparent weight changes as a function of microbalance inclination allows limits to be established for the weight errors due to changes in inclination. For the Cahn RH balance, this limit can be as large as 600 mu g mrad^-1. In relation to specimen temperature measurement, uncertainties in thermogravimetrically-derived rate constants for alloy oxidation resulting from uncertainties in temperature are considered, and techniques to minimize these uncertainties are discussed.

Alignment of Ebert-Fastie monochromators can be easily and rapidly done using the method described in this note. This method is mainly based on the use of a small He-Ne laser and the observation of the Fresnel diffraction pattern. This method can easily be extended to the alignment of Czerny-Turner monochromators.

The atmospheric-ozone measuring Dobson instrument is shown to have defects in optical stopping and focusing which explain the so-called 'directional effect', the dependence of its measurements on incident ray direction. The stopping fault is due to an incorrect lens focal length and may not affect all instruments. The focusing fault is a design problem common to all instruments and its effect has been calculated for a tungsten-type lamp and for direct sunlight. Neither fault existed in the original 1931 design. Their remedy appears to be straightforward.

A plasma gun similar in geometry to the washer plasma gun has been operated with gas injected externally. Hydrogen, nitrogen and argon plasmas have been ionised and accelerated to velocities of the order of 10⁷ mm s^-1 and densities 10¹¹ mm^-3. Higher parameter range is possible with higher electrical input power.

An automatic slow speed spinner magnetometer is described in which the specimens are pre-oriented in cubical holders before being inserted into a magazine. The instrument, presently designed to handle 12 specimens per run, changes and measures the specimens according to a procedure in which each of the three components of the magnetic vector is measured four times. Once the instrument has been loaded and started up, it can be left unattended for the rest of the run. The total measuring time for 12 samples is approximately 55 min.

A method of adding an analogue X-Y recorder to a serial computer terminal is described. No modifications are required to either the computer or the terminal and high level language software may be used to control the graph plotting.

Until recently, time resolved measurements of X-ray scattering patterns have not been feasible because laboratory X-ray sources were too weak and detectors unavailable. Recent developments in both these fields have changed the situation, and it is now possible to follow changes in X-ray scattering patterns with a time resolution of a few ms. The apparatus used to achieve this is described and some examples from recent biological experiments are given.

Design details, construction and performance of a back reflection camera are presented. The design permits short exposure times with total enclosure of primary and scattered X-ray beams.

A nuclear magnetic resonance (NMR) machine capable of producing tomographic sections of the whole human body in vivo has been constructed. This system is based on a four coil, air-core electromagnet producing a field of 0.04 T which corresponds to a proton NMR frequency of 1.7 MHz. The images are produced line by line using a selective excitation technique. Magnetic field gradients up to about 5 mT/m are employed. Electronic subsystems are described here including a radiofrequency (RF) amplitude feedback circuit, an RF power amplifier, a transmit/receive switch, a receiver pre-amplifier and gradient coil drivers. For a single scan through a 10³ mm³ sample of human muscle tissue in vivo, the measured proton density uncertainty is 24% and the spin-lattice relaxation time (T₁) uncertainty is 74%. Phantom images using CuSO₄ solution and in vivo sections through human chest, thighs and head are presented. T₁ measurements of human muscle, liver and brain tissue in vivo give results which agree well with T₁ values for corresponding rabbit tissues measured in vitro.

A digital pulser for Fourier NMR spectroscopy employing TTL integrated circuits is described. Various sequences of 90 degrees and 180 degrees pulses and interval times can be computer programmed. Pulse lengths are set manually with a four-digit thumbwheel switch, the minimum time step is 0.1 mu s. The pulser is phase-locked with a maximum pulse jitter of 3 ns to the RF source frequency of 10 MHz.

A design is given for a circuit that produces an accurately bi-symmetric modulating field of variable frequency amplitude and mark-to-space ratio with sufficient power for most NQR applications including nitrogen resonances in field-gradients of high asymmetry.

The technique allows high precision measurements to be made at high frequencies where disadvantages inherent in the use of acoustic delay lines are significantly reduced. It is possible to make absolute measurements of sound velocity at 50 MHz with errors as low as +or-0.01% using the apparatus described. Thermal instability and the quality of acoustic coupling to the liquid metal limit the precision of measurement at high temperatures. Sound velocity measurements for liquid indium in the temperature range 440 to 919K are presented.

A simple instrument for low-energy ion scattering studies of surface composition and structure is described. Ions are analysed by a hemicylindrical mirror analyser which is also used for Auger electron spectroscopy. The ion source, typically used for 1 keV He⁺ or Ne⁺ ions, incorporates a quadrupole mass filter based on a commercial mass spectrometer. In its simplest form, the complete ion scattering facility can be fitted to a single 150 mm diameter flange compatible with most surface science instruments. The capabilities and limitations of the instrument are demonstrated with results for oxygen adsorption on Cu(100) surfaces.

The position sensitivity of hot-film shear probes was studied with the use of pulse-echo ultrasound. The ultrasonic probe was placed opposite the hot-film probe across a rigid tube. The accuracy of measuring hot-film probe position by ultrasonic methods was studied with the aid of a mechanical micrometer. A similar study was conducted in vivo on the thoracic aorta of dogs to maintain flush conditions between the hot-film probe and arterial wall. The sensitivity of hot-film probes to position with respect to a rigid wall was studied. Probe position was determined by ultrasonic methods. The results showed a 20% deviation in hot-film anemometer bridge voltage with a change in probe position of 0.3 mm from the wall. This could result in a 50% error in computed wall shear stress. Resolution of the method of ultrasonic positioning of the hot-film probe was determined to be 0.1 mm.

Recently developed methods of spectral analysis for randomly sampled signals are applied to data acquired by a laser Doppler anemometer (LDA) operating in the burst mode, where the velocity of individual particles is computed. The measurements, which were obtained in a seeded air jet, support the adoption of a Poisson model for the distribution of the particle arrival times. Spectral estimates formed from the LDA data are found to agree well with corresponding estimates obtained from a hot wire probe, and have a variability which conforms with theoretical predictions. The advantages and limitations of a simple method of reducing the variability of the spectral estimates is discussed.

The performance of a germanium acousto-optic modulator, to provide a frequency shifted local oscillator in a CO₂ laser velocimeter, has been investigated. Good heterodyning efficiency was found in contrast to previous reports. Practical operation of the device for direction sensing is described.

Triple-sensor probes and corresponding analysing methods of the probe's signals are proposed for determining the mean velocities, Reynolds's stresses and all higher-order correlations. From the theoretical procedure no approximation of the quantitative investigation of the flow parameters and no limitation to any turbulence intensity exist in these analysing methods. Restrictions of the probe geometry only occur at very high turbulence intensities.

A water vapour sensor based on absorption by phosphorus pentoxide and simultaneous electrolysis has been developed. The active element consists of a small alumina substrate with a pair of rhodium electrodes. The phosphoric acid coated element is mounted on a common type vacuum flange. The sensor can be used for the measurement of the absolute water vapour pressure (10^-3-10² Pa) in atmospheric and vacuum systems.

A passively Q-switched Nd:YAG oscillator has been developed as a driver for the Soreq High Power Laser System. It is time-tunable with the ability to generate pulses over the range of 2 to 17 ns. It exhibits excellent energy reproducibility (<2%) with no misfires.

A simple, inexpensive temperature controller is described. This controller is capable of maintaining temperatures constant to less than 100 mu K in a variety of thermostats. It uses a thermistor in a DC Wheatstone bridge. The simplicity of DC detection allows the use of readily available electronic components, and ensures a low total cost. Furthermore, thermostatting of the electronics is generally not necessary.

A special Mossbauer furnace suitable for amorphous alloys is described. This furnace allows very rapid heat treatment, and is particularly convenient for the study of the thermal stability of such alloys.

A system for measuring currents from insulating materials under high fields has been developed to allow the determination of complex permittivity by modulating the electric field with a sinusoidal low frequency component (10^-4 Hz<or=f<or=1 Hz). The voltage source is an electrostatic generator with maximum DC output voltage of 80 kV. When it is modulated the maximum DC output is 12.5 kV with a 3.7 kV peak to peak modulation. Data acquisition and processing is under the control of an on-line computer.

A differential pulse transformer with a rise time of less than 2 ns and a droop of less than 1% for pulse widths up to 200 ns is described. The device was primarily designed for the determination of small voltage differences in pulsed experiments on hot electrons in semiconductors. Special attention is paid to the systematic and random errors which may arise at the determination of differences.

The impact on the total transparency of field inhomogeneity around the grid wires of a plane retarding potential analyser (RPA) is taken into consideration at normal incidence of the ion flux. The RPA transparency is determined mainly by the suppressor grid's geometry and attractive potential and not so much by the analysing grid's repulsive potential. Whereas in a floating potential regime the suppressor grid transparency differs for different ion species, in a retarding ion regime it is the same for all. However, at all swept voltage values of the analysing grid the total RPA transparency remains dependent on the ion masses and the grid potentials.

An apparatus is described in which the sliding friction between a small rider and a flat surface was accomplished. Normal load was applied to the rider without mechanical contact by an electromagnetic loading system. Friction force, normal displacement of the rider and electric contact resistance could be measured simultaneously. The apparatus may be used widely to study the dynamic motion of sliding bodies under miscellaneous frictional conditions.

A switched-mode DC sputtering power supply suitable for powering magnetron targets is described. It uses an output L-C-L filter with fast-acting feedback to limit arc-over energy, and operates at 20 kHz to give a fast response time and a small size and weight. High peak currents are delivered from this supply compared with more conventional power supplies during arc-over, but the total arc energy is low in a continuously arcing discharge. The supply described delivers 1 kW and weighs about 6 kg. Higher output powers could be obtained with additional switch elements and output networks.

A Michelson type interferometer designed for the accurate comparison of different wavelengths with the primary krypton length standard is described. Its design concept, constructional details, principles of operation and performance are discussed. The instrument is disposed in a vertical plane and the movable mirror is adjustable into either one of two accurately reproducible, discrete positions. No mechanical forces are exerted on the interferometer structure either by the shifting of the mirror or by its adjustment mechanism. In a first series of measurements with the krypton 86 standard lamp as the light source, the standard error of the mean was 1.1 parts in 10⁹.

The design procedure described generates alternate designs to facilitate the selection of the optimum.

The author discusses the way in which the instrument described by Lue (ibid., vol.12, p.833, 1979) can be upgraded with no serious modifications.

The author briefly describes the work of the group at RSRE Malvern in the field of photon correlation.