Table of contents

Two automatic control systems for filling large tanks with gravitationally stable density stratified liquids are described. They are: (i) an analogue method using gate and mixture control valves operated through servomechanisms by rotating cams; (ii) a digital method with a numerically controlled metering-type pump. Measurements taken of a typical stratification showed that the actual density distribution was within 0·02% of that expected using the analogue method and within 0·01% using the digital method.

In order to study certain wave-motions which occur in the two-phase flow of a gas and liquid along a tube, a simple repetitive scanner was devised. It consists basically of a pivoted mirror which is rocked by a suitably profiled cam, turning with constant angular velocity. The device can be applied in the measurement of velocity and in the viewing of any object moving at constant velocity along a straight line.

New techniques applicable in the design of remote recording wind vanes are presented, and two preferred systems are described in detail. A small Duralumin vane is coupled to the shaft of a continuous rotation potentiometer, which may be a high resolution wire wound instrument, or constructed from a low-torque rotary switch. In one system the potentiometer has provision for alternative inputs, the supply voltage being automatically switched from one input to the other following a positional triggering action of the moving contact. Another technique utilizes a step potentiometer with one supply position, the sign of the output reference voltage being automatically changed as the wiper traverses the input connections.

The outputs of the transducers change progressively over ranges of up to two revolutions of the shaft, and when range extremities are reached, revert to voltages which correspond to angular changes of 360°. The overlap provided prevents an incoherent recording which would otherwise be obtained when the moving contact reaches the ends of the track.

A complete instrument weighs 350 g, has a vane 25 cm long and operates in winds down to 25 cm s⁻¹ and the systems are well suited to automatic weather station operation.

The dynamic response of constant resistance anemometer systems is related to the thermal equilibrium of the probe during unsteady changes in surface cooling and also to the characteristics of the feedback system which supplies the heating current. This maintains the probe resistance and average temperature at approximately constant values. In this paper the thermal equilibrium and feedback equations are combined, taking account of the inductance of the probe cables, to derive the governing equation for the unsteady probe current. The probe inductance raises the order of this equation from first to second.

A calibration method is described for a five-hole spherical Pitot probe by which the direction of a velocity vector is fixed by two Cartesian angles, one lying in the horizontal equatorial plane and the other in a vertical meridian plane containing the velocity vector. Measurements are found to be possible within the range of up to 65° half cone angle from the probe axes.

In a Fabry-Perot interferometer of the order scanning type, the optical path is kept constant and the interference pattern is scanned in a radial direction with a narrow slit. A simple experimental method is described to determine the dispersion function of this spectrometer with a relative error of less than 0·5%. For Lorentzian and Gaussian spectral line profiles, the influence of the form of a rectangular detector slit on the shape of the measured line profile is discussed. Numerical computer calculations of this influence for the case of a Lorentzian line profile are presented.

The sensitivities of five transistorized nuclear magnetic resonance spectrometers have been measured: a Q meter, a Robinson oscillator, a common marginal oscillator, a modified PKW oscillator and a marginal oscillator the limiter of which was placed behind an r.f. amplifier. All the spectrometers except the common marginal oscillator appeared to possess the same sensitivities for all considered oscillation levels and modulation frequencies, provided that the PKW oscillator and the second marginal oscillator are made sufficiently critical. The common marginal oscillator was less sensitive as this spectrometer could not be made very critical, which resulted in a large contribution of the 1/f noise.

A determination has been made of the efficiency with which certain commercially available channel electron multipliers detect H⁺ and Ar⁺ ions. The three multipliers tested were Bendix types 4013 and 4039, which have 3 mm and 10 mm diameter funnels respectively, and a Mullard type B419 BL which has a 10 mm diameter funnel. The efficiency shows saturation for ions with energy above a few kilovolts and is relatively constant across the whole surface of the 10 mm funnels. The particular 3 mm funnel tested shows an energy dependent sensitivity across its surface.

An electron beam evaporator for shadowing electron microscopic specimens with refractory metals is described. Details of the construction and the mode of operation of the shadowing gun and the power supplies are given. The apparatus is designed to keep the thermal load on the specimen, during shadowing, lower than other methods but at the same time to give higher resolution. The method is therefore especially suited for application in the freeze etching technique, using a tantalum-tungsten alloy as the shadowing material.

A three-parameter equation is found which gives temperature as a function of resistance for Speer 470 Ω, ½ W carbon resistors with an accuracy to fraction one-third % over the range 0·6-4·2°K; the parameters can be determined from data at three calibration points. If separate equations, each with three parameters, are used for the ranges 0·6-2·2°K and 2·2-4·2°K the accuracy is everywhere better than 2 mdegK

An apparatus has been designed and developed to study low frequency internal friction in neutron irradiated foils in the temperature range 77-300°K. The pendulum is specifically designed to have a single mode of oscillation with negligible dissipation of energy at higher parasitic frequencies. It is driven automatically and the amplitude is maintained constant by a controlled gain positive feedback loop from which a continuous recording of internal friction of the specimen is obtained. A typical example of the damping of neutron irradiated pure iron is given showing a complex peak in the temperature region 200-240°K.

In this paper we describe improvements to the experimental and computational methods used in the calibration of a magnetoresistor required for the measurement of magnetic fields in a hysteretic high-field superconducting magnet. The de Haas-van Alphen effect is used to interpolate between nuclear magnetic resonance calibration points in order to increase the accuracy of the calibration whilst reducing the experimental effort. The accuracy of these results is limited by the instrumental resolution which is 9 G at 100 kG. The current measurements have provided the most precisely known dHvA frequencies to date for the <111> neck and belly orbits of the Fermi surface in gold.

Two identical multi-layered solenoids each enclosing a water jacket are used in an a.c. bridge circuit so that inclusion of a sample in either coil produces an out of balance signal proportional to the susceptibility of the sample. This signal is then amplified with low noise field effect transistors, converted to direct current with a phase sensitive detector, and displayed against temperature on an XY recorder. The water jacket of one solenoid contains a furnace capable of reaching 700°C, while the other contains a cooling assembly for use between liquid nitrogen and room temperature. The temperature variation of susceptibility allows determination of Curie and transition temperatures.

A description is given of an air thrust bearing attachment which, when applied to two dead weight gauges, converts the system into a direct reading recording differential manometer operational between pressures of 600 and 100 000 kN m⁻². The accuracy is in the range 350-500 N m⁻² (2½-4 torr) for differential pressures in the range 0-5000 kN m⁻². The system is suitable for use with fluctuating pressures.

The behaviour of divided photoelectric cells as position-sensitive detectors in alignment systems is described and expressions derived for the signals obtained when bisected and quadrant detectors are illuminated with a Gaussian (laser) beam. The effect of the gap between the segments of a detector array is considered. Two methods are suggested for obtaining independent signals for displacements in two orthogonal directions. The performance of a typical bisected cell is reported and estimates of the sensitivity and absolute accuracy are given.

The high resistance encountered when making electrochemical measurements in non-aqueous solvents, necessitates electronic control of the electrical parameters. An instrument is described, which performs the current reversal, used in cyclic-chronopotentiometry, electronically, thus permitting good regulation of current during individual transitions and rapid current reversals between transitions. Current can be preset within the range 1 μA to 60 mA and its regulation is better than 1%. With transition times of 20 ms, the dead time is less than 0·1%.

A method is described whereby the relaxation times of exponential traces recorded on 35 mm film are measured by direct comparison with an exponential curve of variable but known relaxation time, displayed on an oscilloscope screen. The new technique reduces the time required for the evaluation of such traces to one third that required by the conventional technique, and is accurate to ±1%.

An automatic translation balance for recording the temperature variation of magnetization of rocks is described. A beam is suspended horizontally so that it is free to move only in the direction along its axis. The deflection produced by a magnetic field gradient acting on about 70 mg of rock is detected by a novel capacitance sensing device. Compensation is effected by a feedback mechanism which produces a voltage proportional to the force experienced by the beam. The balance is capable of sensing a force of about 2 × 10⁻⁷ N. An XY recorder is used to display the output of the balance against the sample temperature.

The conditions which govern the spacing of the periodic markings which develop on {110} planes in Al single crystal during electropolishing have been examined. For spacings in the range 480-900 Å a linear relation is obtained between the spacing and the time of polishing. It has been shown that the periodic markings can be used as a grazing-incidence diffraction grating for long wavelength x-rays such as C-K α and Be-K α radiations.

A simply constructed spark source is described, capable of producing intense, predominantly ultra-violet light pulses of risetime 3 ns, half-width 7½ ns. The source can be triggered with a jitter of ±3½ ns, or allowed to free-run. The design permits rapid adjustment or renewal of the electrodes and positioning of the focused spark under conditions of high ambient lighting.

The magnetic susceptibilities of several weakly magnetic materials commonly used in the construction of cryogenic apparatus have been measured between 4·2 K and 1·4 K and the results are given to a much higher accuracy than previously known.

Some modifications have been carried out to the NPL designed standard thermometer calibration baths to improve the ease of their operation.

Predetermined temperatures can be reached readily by incorporating a proportional temperature controller. To obtain the required rapid response characteristics, the heaters are now immersed directly in the working fluid and the thermal capacity of the system has been considerably reduced. Adequate temperature stability and uniformity have been achieved.

In this note a new type of skin friction balance is described. The elemental surface in the balance is floated on an air bearing and the skin friction force is resisted by a spring, the deflection of which is determined by a position sensor. The balance may be adaptable to different surface geometries and used for unsteady as well as steady flows.