Table of contents

Instruments can be considered as systems, and there are only a few structural schemes employed in the construction of such systems; one such scheme is the use of feedback. The general mode of operation of feedback-measuring systems is ascertained, and the reasons for applying feedback to a measurement situation are established. Using feedback it is possible to improve accuracy and speed of measurement, reduce the effect of interfering and modifying inputs, and allow remote indication and non-contact measurement. The property of inversion assists instrument design, and can provide for digital indication. Basic methods of stability assessment are given. The common balance variables are listed, and a wide range of instruments is discussed, making explicit the general properties of feedback. Measuring devices having a frequency output maintained by feedback, and the use of feedback for control purposes in instruments, are considered. The pedagogic implications are referred to, and an extensive list of references is provided.

Substrates for film deposition can be cleaned and/or modified in a variety of ways by various sputtering techniques, but this can be done controllably only by careful attention to all of the possible effects that can occur. In this review the known effects are summarised. Some of the system configurations that can be used are described. The physical and chemical sputtering effects are emphasised. These include rates of material removal and recontamination, backscattering, redeposition, trenching, faceting, cone formation, formation of altered surface layers, dissociation of compounds, diffusion, sublimation, ion implantation, crystallographic damage and radiation damage.

Describes a new design of resonant spectrophone for the analysis of gaseous samples in which the authors have attempted to optimise the collection of the acoustic waves and the coupling between acoustic modes and the laser beam. They have thus reached a good sensitivity (below 5 parts in 10⁹ for NO₂) with a fixed-frequency laser and relatively simple and inexpensive components.

A new high-pressure cell for depolarised light scattering from compressed fluids is described. It consists of a thin-walled inner cell inserted in an outer pressure cell. The inner cell is formed by a glass tube which is covered on one end by a membrane filter. This filter is used both for cleaning and filling of the inner cell. It also serves to establish the working pressure in the inner cell by raising the pressure in the outer cell, which is also filled with the test liquid. Dust-free pressurised samples can be obtained in this way, and parasitic stray light from cell windows is largely reduced.

A simple linear ohmmeter for temperature measurements by negative-temperature-coefficient (NTC) resistors up to 6 k Omega is described. As very low power (less than 0.6 mu W) is consumed by the NTC resistor during resistance measurement, only negligible self-heating of even very tiny thermistors is produced. Resistance readout can be performed by means of a digital dial knob (by a null method) or by measurement of the voltage offset from pre-set zero. A reading accuracy of the order of 1 Omega was obtained.

An extensometer for axial strain-controlled low-cycle fatigue tests on anisotropic structural materials at temperatures up to 816 degrees C is described. The unique features of this extensometer are that it is designed to be supported by counter weights when attached to a uniform-gauge specimen, and that the probe tips are held in place by friction. Results of tests on austenitic stainless steel weld-metal specimens indicate that the extensometer provides accurate cyclic strain control and has excellent thermal and electronic stability for tests of up to 4.5*10⁶ s duration.

An automated bent-strip method for assessing the internal macrostresses in electrodeposits is described. True null deflection of the strip is maintained and an electrical output is available which can be recorded continuously, and processed. A modified calibration technique is described.

The results are presented of an experiment to investigate the feasibility of using a vacuum arc discharge as a source of C⁺ ions.

A general-purpose time-proportioning regulator implemented with only very common and cheap electronic components is described. The regulator, operated at about 1 Hz, together with a silicon transistor thermometer, was used as a temperature regulator: (i) in a commercial 51 circulating liquid thermostat the temperature was stable to within 0.1 degrees C during 1 h, and no overshoot was observed after a 10 degrees C step; and (ii) in a home-built microscope hot stage the temperature stability was better than 0.01 degrees C.

Formulae are developed to describe the nonlinearity and the positional resolution of a position-sensitive counter in which the output charge from a microchannel plate is shared between four collecting electrodes. Performance figures are expressed in terms of the positional spread of the electron charge from the microchannel plate.

Platinum resistance thermometry has been used for temperature measurement in situ in a high-pressure vessel up to 360 MPa. Commercially available 100 Omega platinum resistors have been calibrated against pressure for temperatures between -60 and +100 degrees C. A convenient small four-wire electrical connector has been developed for use up to 500 MPa.

A technique for the measurement of frequency within a cycle of a periodic input is described. This can be useful for quicker measurement of low frequencies.

A device for measuring the mechanical properties of polymers in shear has been devised, built and tested. The type of deformation employed is that of simple shear of flat sheets and the displacements are measured using a Hall-effect extensometer. The method has been tested using isotropic samples of low-density polyethylene and the results for shear compliance are found to agree satisfactorily with those obtained from conventional extension tests. Continuous records of shear strain or compliance against time have been obtained, as have measurements of the variation of compliance with strain. The method is quick and simple and gives direct results for shear compliances, time dependences and strain dependences, the last two properties being very difficult to measure using conventional torsional techniques.

The theoretical response of an oxygen-reducing copper cathode to saline velocity is described for a range of electrode shapes and sizes and an experimental study of various velocity magnitude transducers based on the effect is presented. A preferred form of transducer is suggested which is small, requires one-sided access to low-volume solutions and has a symmetrical square-root response to velocity magnitude covering typically the range from 800 to 1 mm s^-1. The time dependence of the response and other factors limiting the performance of the transducer are discussed.

To evaluate cinematographic films of the growth of slip lines on metal crystals during tensile deformation an inexpensive device controlled by a microprocessor has been designed. During evaluation the system corrects for frame-to-frame movement of the object, measures the photographic density, corrects for the sensitometric curve of the film and integrates across the intensity distribution of the slip line.

A digital electronic control system is described which enables two-component alloys to be prepared by co-evaporation from independent thermal sources. Evaporation rates are monitored by quartz crystal microbalances. The use of digital techniques to process the resulting signals enables true integral control to be achieved, free from the problems of long-term drift, and also enables film composition profiles to be recorded accurately. Results obtained when using the system to prepare amorphous magnesium-bismuth films are analysed, and it is shown that compositional errors can be reduced sufficiently to be dominated by noise in the crystal circuits and by calibration errors. With this system, magnesium-bismuth alloy films with compositional variations less than +or-0.5 at.% have been routinely prepared, and there is no reason to expect that this level of control cannot be achieved in other similar alloy systems.

A fully automatic 'magnetic cleaning' device suitable for demagnetisation of igneous rock specimens is described. The apparatus consists principally of a two-part solenoid energised through an autotransformer, a two-axis tumbler which is divisible into two parts, and an astatic magnetometer giving the three orthogonal magnetic components of the partially demagnetised rock samples.

The development of pyrometry with time resolution in the microsecond range is required by high-speed thermophysical property experiments with millisecond duration. The general design criteria of a high-speed pyrometer with such a resolution are discussed and the contribution of several parameters is analysed to optimise the pyrometer performance with respect to the temperature-time characteristic of the event to be measured. Salient features of a pyrometer built on the basis of these criteria are described, with the details of the procedures necessary for its calibration. Monochromatic operation either near 0.65 mu m or near 1 mu m and a response time to a step input of 5 mu s make the instrument adequate for measuring temperature with a time resolution in the microsecond range. Emphasis is placed on the inaccuracy of temperature measurements made with the instrument.

A method and an apparatus for measuring the dispersion characteristics of thin-film metal-insulator-metal structures at frequencies below 1 Hz are described. Application of this apparatus makes it possible to study the complex permittivity of thin films having a loss tangent of the order of 10.

A pulse interferometer, making use of the phase comparison principle, is described which uses monolithic and discrete solid state devices for determining ultrasonic velocities in solids. The apparatus is easy to construct and operate. The pulsed radio frequency output from the final power amplifier is 60-80 V peak-to-peak. Absolute ultrasonic velocities in solids can be measured to an accuracy of 1 in 10⁴ and relative velocities to 1 in 10⁶. The constructional details of the circuits are discussed.

In a previous paper (Silvester and Johnston 1976) the authors described an electro-optical curve follower which used analogue feedback control to steer a four-quadrant photodiode detector along a bright line on a screen. By using a PDP11/10s minicomputer (Digital Equipment Corporation) with 16 k of store and an AR11 interface unit they have been able to dispense with the data logger and analogue computer.