Table of contents

A method is described for the determination of bulk volumes of granular solids by mercury pycnometry. A special pycnometer made of glass is used which has only one inlet without any taper-joint portion. The weights of mercury filling the pycnometer are corrected for the deviations of mercury surface at the inlet. The above improvements allow bulk volumes of samples to be measured within +or-0.0001 cm³.

With the novel mercury microvolumeter rapid and accurate measurements of small volumes of low-density highly porous substances can be made. This instrument is well suited to the determination of the apparent density of fine highly porous particles in a powder and it eliminates the problems normally associated with measurements on such materials. The device is simple and inexpensive and may be constructed from materials commonly available in the laboratory.

The design and construction of a 90 degrees electrostatic prism is described. The device is used to deflect hypervelocity dust particles produced in a horizontal van der Graaff accelerator to simulate micrometeoroid impacts on dusty lunar and asteroidal surfaces where vertical incidence must be provided.

Measurement of the tensile strength of powder compacts is useful for determining the magnitude of the cohesive forces which cause powders to agglomerate. A split-cell apparatus has been developed which contains major improvements on previous devices, including separation of the split-cell from the base unit to allow higher packing densities. A piezoelectrically controlled translation device provides a means of vibration-free, variable speed application of the tensile load. A compact DC operated force transducer is used to measure the tensile load to an accuracy better than 0.1%. Several other design and procedural improvements also make the present method simpler and more efficient. Typical plots of tensile stress against time and tensile strength against packing density are shown for carbon black.

A simple multiscaler interface for Mossbauer spectroscopy has been constructed for the Apple II microcomputer. The interface collects two simultaneous spectra to allow the use of a double-end spectrometer.

An apparatus is described for surface cleaning and sample addition for the measurement of physical properties of insoluble molecules at the air/water interface. To clean the surface the subphase liquid level is increased to engage aspirators positioned above the surface. These also adjust the liquid level by reproducibly breaking contact with the liquid during aspiration. Sample is added via an autosampler of the type used in high-pressure liquid chromatography. The advantages of the apparatus are the reduced chance of surface contamination and oxidation. When used in conjunction with a computer-controlled film balance, the system can be fully automated with a 25 sample capacity.

Solid-state voltage references are much more robust than conventional electrochemical Weston standard cells. This factor is important in transferring voltage from one laboratory to another over large distances. The response of a commercial solid-state voltage reference to changing environmental conditions is examined, and a useful modification is suggested to reduce the uncertainty of measurement. Results are reported for temperature, pressure and vibration tests as well as its performance during a sequence of international voltage transfers.

It is shown that suitable HV diodes can be used as variable HV resistors or switches.

A sensor of small size and low profile (5 mm) has been developed for the detection of movement and instantaneous range of remotely located objects. The sensor operates as a short-range Doppler radar by producing microwave oscillations in the X-band frequency range. The construction uses RT-Duroid, a planar microstrip transmission line medium. A Gunn diode is used as the active device, with a self-biased Schottky mixer diode acting as the receiving element. The microstrip circuit comprising the sensor is described. Receiver sensitivity is presented for the principal antennae axis and measurements of mixer signal-to-noise ratio as a function of electronically generated Doppler return signals are given. The sensor has been used in the detection of man-targets at ranges in excess of twelve metres as part of an advanced microprocessor-based alarm system.

An inexpensive versatile system for x-y scanning of a laser beam is described. Complex patterns may be scanned rapidly and accurately at a selected speed that is effectively constant regardless of direction changes. An Apple II microcomputer creates files for desired patterns and controls two galvanometer scanners in a stepping mode. Undesirable spatial variations in target heating caused by the stepping action are limited by controlling the step parameters. These variations are briefly examined theoretically.

Thyratron switching of the electric excitation of transversely excited atmospheric pressure (TEA) nitrogen lasers results in higher energy, about four times better stability (3% FWHM) and 100 times smaller jitter (1 ns) compared to spark gap switching. The laser operation is very sensitive to the heating voltage of the thyratron.

Standard measurements of infrared light-emitting diode (IR-LED) device parameters with simple optical bench apparatus and inexpensive electrical equipment are described. Testing circuits and mechanical mountings are given in detail. The influence of the PIN detector device on the results of switching time testing is discussed. A survey of device parameters of commercially available LEDs is presented (efficiency eta , radiant flux Phi _E, angle of half intensity of spatial distribution alpha , risetime and falltime, t_r and t_f, for constant forward (pulsed) current I_F).

A gas flow cryostat is described in which the real and imaginary parts of the complex relative permittivity of a solid dielectric may be measured at any temperature between 14K and 273K. The leads to the electrodes in this cryostat have a sufficiently low inductance to allow measurements up to a frequency of 2 MHz without correction. The temperature may be controlled electronically or manually with a best accuracy of +or-0.01 K. The cryostat is of simple construction and easy to use. The results of dielectric measurements of CaF₂:Er³⁺0.15% are included as an example.

The theoretical principles and practical design for the linearisation of resistance temperature detectors (RTDs) by feedback are described. The linearisation is equivalent to driving the RTD from a voltage or current source with the correct impedance and the technique may be applied to any resistance transducer. Emphasis is placed onto the platinum resistance thermometer (PRT) and the evaluation of residual nonlinearities. The residual nonlinearity of a PRT linearised over the range -40 degrees C to 200 degrees C is less than 0.02 degrees C.

A detailed root-locus analysis of the exact second-order model of the constant temperature detector bridge feedback system developed previously (Sherlock and Wyatt 1983) is presented. Such systems are widely used in hot-wire anemometry and have applications in other areas such as bolometric radiation detection. The results of this analysis clearly identify the bridge offset voltage as a most important and practically useful control parameter for tuning the system response to a desired form. The analysis also identifies a characteristic frequency omega _j= alpha I_D²/C_th (where alpha is the temperature coefficient of resistance of the detector element, C_th its thermal capacity and I_D the quiescent current through it) as a crucial measure of the potential for bandwidth enhancement. If omega _j/> omega _i, the intrinsic thermal cut-off frequency of the detector, and omega _e is the bandwidth of the amplifier in the bridge feedback system, then it is seen that appropriate tuning of the bridge offset gives a critically damped system with a bandwidth approximately ( omega _e omega _j)¹2/.

A rapid and accurate method is presented for measuring the transfer function of seismic channels by using the step-excitation technique. The parameters of the Laplace polynomials are estimated by inverting the step response of the channel. Special attention is paid to the estimation of reliable confidence bands: the statistical structure of the observed residuals is taken into account. An example is given for S-5007 long-period instruments coupled to four-order bandpass filters. Accuracies of one per cent and one degree for the amplitude and phase response respectively are obtained in the passband by stacking a great number of identical pulses. Advantages of the method are analytical transfer function description, high accuracy and dependable error estimation.

Using the experimentally measured axial flux density distributions of asymmetrical electron lenses, some of their electron-optical properties were studied in detail over a wide range of asymmetry under nonsaturation conditions. The limitations of some relationships concerning the maximum flux density and the halfwidth of the axial magnetic field of asymmetrical lenses were discussed. Empirical equations describing the chief properties of the flux density distribution were put forward. The effect on the electron-optical properties due to the transfer from the double pole piece asymmetrical lens to the more simple design of the single pole piece lens was investigated. The results should provide a better understanding of some of the electron-optical properties of asymmetrical double pole piece and single pole piece electron lenses which at the present time are widely used in various types of electron microscopes.

Describes a new kind of a particle gun called the hybrid gun. It is able to deliver a finely focused electron or ion beam simply by reversing the polarity of the acceleration voltage. The detailed design features of the gun are given and the electron-ion optical properties are discussed. For typical operation in surface analytical instrumentation the hybrid gun delivers in the electron mode at 3 keV acceleration voltage a beam current of 30 mu A in a spot of 200 mu m FWHM and in the ion beam mode at 2 kV, with an ionisation voltage from 200 to 400 V, a beam current of 0.5 mu A to 3 mu A in a spot of 100 mu m FWHM. The applications of the new gun in surface analytical instrumentation are briefly discussed.

The design and performance are described of a simple and inexpensive rotating barrel to ion plate aircraft fasteners and small components. Pure aluminium, controlled-composition Al-Zn alloy and Al-aluminium oxide coatings were ion plated onto aircraft fasteners using the rotating barrel. Highly dense coatings were obtained even at low argon discharge pressure ( approximately 0.133 Pa) by increasing the power density using thermionically assisted triode discharge. Because this process is not confined to line-of-sight deposition, parts with complex shapes can be ion plated with a reasonably uniform coating. Thickness uniformity can be varied for each portion of the fasteners simply by changing the argon discharge pressure without the need to alter the power density or the grain structure.

The authors have performed differential measurements of fluorescence lifetimes by using photomultipliers with matched single-photon response for simultaneous acquisition of the fluorescence and excitation. This 'SAFE' technique automatically corrects for any change in the optical pulse during the experiment, thus eliminating a common source of systematic error. Preliminary results obtained using a standard reference of PPO in ethanol give a lifetime of 1.63+or-0.02 ns which is in good agreement with that obtained using conventional single-channel fluorometry.