Table of contents

There are many types of measurement which are upset by rapidly changing electric or magnetic fields. Such interfering fields are often created by switching on and/or switching off loads across the supply mains. A circuit is presented which prevents the creation of large transient fields at switch on by delaying connection of a load until the first zero of the supply voltage after closure of the contacts CC. Similarly, switching off is delayed until the current through the load falls to its first zero after opening the contacts. The circuit is useful as an interface between low-power logic and apparatus controlled by it. It has been found very useful in suppressing interference when making dielectric measurements, in preventing transients causing false counts in digital apparatus and in many other applications.

Mechanical, structural and geometrical properties of metals, such as the presence of defects, hardness, degree of heat treatment, and dimensions can often be tested nondestructively by electromagnetic methods. These methods include the use of eddy currents and magnetic particles, which are particularly effective for locating and sizing surface and subsurface defects, and the application of magnetic hysteresis. The established techniques and some of their more recent developments are described.

The temperature-jump technique is one of the most widely used methods for measuring the rates of those chemical reactions in solution which are so fast that reaction is complete as soon as the reagents are mixed. The reaction mixture is allowed to come to equilibrium, and then the equilibrium is perturbed by a rapid temperature rise, produced by Joule, microwave or laser heating. The rate at which the system adjusts to the new equilibrium is monitored by observation of the intensity of light absorbed or fluoresced, or by the electrical conductivity. Rate constants as fast as the theoretical limit of 10¹⁰ dm³ mol^-1 s^-1 are measurable. The review considers the limitations on the performance of temperature-jump apparatus and gives examples of systems studied.

A trigger circuit on the basis of 'Vertical' MOSFETs is described. It gives a 1.5 kV voltage step with 15 ns risetime at the cathode of a EG&G grounded grid thyratron. The thyratron is incorporated in a transverse-discharge nitrogen laser. The jitter in laser pulse emission, i.e. thyratron firing, is restricted to less than 1 ns.

A simple and inexpensive peak-and-hold circuit for short laser pulses is described. This system consists of two basic units: a peak-and-hold circuit and a sample-and-hold. By using this system, one can measure the peak height of a laser pulse with an accuracy of 0.1%, if the pulse width of the single-shot laser is more than 50 ns.

The authors present a simple technique for measuring spectral distribution of nanosecond optical pulses using a multichannel system consisting of an array of optical fibres, simulating the conditions of Thomson scattering in plasmas. The advantages and limitations of spectral and temporal resolutions relevant to any form of pulse spectra are also analysed.

The authors describe the design and construction of an absolute spectral reflectance attachment for a commercial spectrophotometer. It is designed to measure the spectral reflectance of coated samples from 185 nm to 850 nm wavelength. Examples of spectral scans of aluminium and silver films measured with the attachment are shown, and compared to published values of reflectivity at various discrete wavelengths.

A modification of Hall effect measurement for thin films by the so-called three-terminal network is described. It is based on a special geometrical arrangement of Hall electrodes using photolithography and lift-off techniques.

A method is presented for measurement of electrostrictive deformations of ferroelectrics at high temperatures, using a capacitance transducer and a straightforward detection system. The system developed was used to determine the temperature variations of the electrostrictive coefficients Q₁₁ and M₁₁ for a BaTiO₃ single crystal and ceramic PbZrO₃.

A microwave cavity which can easily produce atmospheric pressure and high flow rate plasmas is described. Atmospheric pressure nitrogen plasmas with 2.5 m³ h^-1 flow rate are obtained with 500 W power supply.

A source of polarised He(2³S) metastable atoms is described providing a thermal energy beam of the order of approximately 10¹³ N s^-1 at a polarisation P_z approximately 95%. The polarisation can be simply reversed using an adiabatic transition, without changing the beam trajectory or its intensity.

Surface tension of a single liquid or interfacial tension between two liquids can be measured by permitting the denser liquid to drain from a reservoir through a vertical tube of diameter about 8 mm which acts as a siphon. As the drainage rate decreases, the meniscus at the base of the drainage tube becomes unstable and thus the siphon effect ceases. The minimum level of liquid in the reservoir during this incomplete drainage process provides an indication of the surface or interfacial tension of the draining liquid. This concept has been tested for sixteen liquids in air and for four liquid-liquid systems.

Crack growth in metals during fatigue testing may be observed using a synchronised xenon flash tube as the illuminator in a metallurgical microscope. A circuit is described which enables the observed opening of the fatigue crack to be selected between the closed and fully open positions.

A dynamic pupillometer which uses a 100-element linear self scanning photodiode array is described. Radiation from IR emitting diodes is used to image the eye. Both digital and analogue outputs of pupil diameter are provided. Accuracy is better than +or-2.5% of full scale and the rise time approximately 20 ms.

An automatic focusing system for medical endoscope cameras has been developed which, when used in conjunction with automatic film advance and exposure control will greatly simplify medical endoscopic photography. A direct autofocus technique is used; this senses the actual image formed by the optical system and attempts to maximise the fine detail. No modifications to the endoscope are required. In comparative tests on realistic test objects the autofocus system is more accurate than manual focusing and is close to the limits set by the Rayleigh criterion.

A volume NMR image can be reconstructed by backprojecting a set of filtered projection profiles onto a 3D matrix. It has been demonstrated previously that the image can be obtained by either a direct 3D backprojection or by an indirect, two-step, 2D backprojection. In this paper, the mathematical equivalence of these different procedures is explained from the geometrical nature of the 3D backprojection. The two-step technique is simply a fast algorithm which groups the summations properly and efficiently in computing the backprojection. It is however applicable only when the main magnetic field is uniform. If the field is inhomogeneous, then direct backprojection must be used in order to obtain an accurate image by a curvilinear reconstruction technique. The linearity of the gradient fields is not a factor determining the applicability of the fast backprojection.

A description is given of a prototype 20 mm diameter medical flowmeter having six electrodes, the effect of which is to halve the variation in response to irregular nonaxisymmetric flow. When flow is axisymmetric the sensitivity rises by 5.2% as the flow Reynolds number increases from 1100 to 17600. The axisymmetric variation is largely determined by magnetic field non-uniformity arising from the short length of the magnet (10 mm), and is practically independent of the electrode configuration. It should be possible to reduce the sensitivity variations for both axisymmetric and asymmetric flow by decreasing the field strength in the electrode regions. Multiple electrode flowmeters have an improved efficiency, and can be used with conventional flowmeter units to which a modification to the input circuitry has been added.

When electrolytic solutions or colloidal sols are available only in small quantities, the immersible electrode system required for conductivity measurements on the solution should be small enough to be compatible with the volume of the test solution. To meet this requirement, a probe is described which is sufficiently small for complete immersion in a test solution of about 2*10^-3 cm³ in volume. This probe consists of a single-piece miniature structure formed by a cylindrical capacitor with a central conductor that leads through a ceramic dielectric and a metallic cylinder which coaxially sleeves the outer surface of the ceramic. A protrusion of the inner conductor and the outer sleeving jointly constitute the two-electrode probe. When immersed in an electrolytic solution or a sol and excited by an AC source, the admittance of this capacitive probe would change which can be measured and related to the solution or sol conductivity. Design considerations involved in the construction of the probe and the associated measuring circuits to get optimum performance are discussed. Experimental results obtained from a test probe are presented and discussed.

Two simple but accurate methods are described for the measurement of thermal relaxation times, both using a modulated IR beam from an arc lamp as the source of thermal excitation. Results are obtained for several metal ion-alanine systems and compared with results obtained using a more conventional T-jump apparatus. Different methods of obtaining lifetimes from modulation methods are compared and contrasted.

A method is described for measuring total expansion of a bulk sample of conducting material, using a microwave resonance technique. The method has been used to measure thermal expansion of niobium and of a sample of beryllium-copper in the range 4K to 300K.

Describes apparatus designed for measurement of volume changes of amorphous metallic materials. The apparatus is suitable for thermal dilatation measurements up to 1300K under isothermal and linear heating regimes, heating rates ranging from 5*10^-3K s^-1 to 5K s^-1, as well as for magnetostriction measurements. Samples are studied in an inert gas atmosphere. The resolution of the apparatus is 2*10^-7 m and 2*10^-8 m for linear heating and isothermal regimes respectively.

Multi (four and eight)-fold path interferometers have been developed for the purpose of dilatometric measurements. The interferometers have two advantages compared to a usual interferometer. One is the self-compensation of the optical misalignment caused by the thermal deformation of the measuring components during the measurement. The other is that no special finish of the specimen surface is required. A very high stability of about 1 nm for eight hours has been confirmed from the experiment.

The rigorous expression for the transmission of a thin absorbing film on a transparent substrate is manipulated to yield formulae in closed form for the refractive index and absorption coefficient. A procedure is presented to calculate the thickness to an accuracy of better than 1% with similar accuracies in the values of n. A method to correct for errors due to slit width is also given. Various formulae to calculate the absorption coefficient accurately over almost three orders of magnitude are discussed. Only data from the transmission spectrum are used and the procedure is simple, fast and very accurate. All formulae are in closed form and can be used on a programmable pocket calculator.

Discusses the practical design and performance of lasers stabilised by balancing the powers in two orthogonally polarised modes. Procedures for aligning the optical system and for optimising the electronic controller are described. The performances of several types of commercial tubes have been tested over periods of over three years. A short-term (1 s) stability of 20 kHz and a long-term (several months) stability of 10 MHz have been achieved with some tubes. An additional characteristic of these lasers is their good power stability (within 0.1% over several hours).

An automatic control and data acquisition system for time-of-flight mass spectrometers is described. The timer consists of a four-channel counter with 6 ns resolution. The operation of the spectrometer is controlled by an inexpensive desk-top computer (HP-85). This computer also facilitates data storage and data output. A reasonably fast repetition rate of five desorption cycles per second can be obtained.

An improved high-voltage low-frequency waveform generator is described. It is based on a design by Miller et al. (1975) with the addition of a feedback control loop. The device can supply up to 27 kV and 2 mA from 0-100 Hz, and up to 10 kV at 1 kHz. Total harmonic distortion is reduced from the earlier device to under 1%, and phase response (under 1 mradian lag at 1 kHz) is good enough for straightforward use in three-phase circuits. Components are inexpensive, with widely available high-voltage triodes used in the output stage. A fibre-optic link isolates the high-voltage sections from the low-voltage control network.

The design of an amplifier with a suitable gain (of the order of 10⁶-10⁸ V A^-1) and a 600 kHz bandwidth is described for EBIC imaging within a SEM. The performance is evaluated with some types of specimens (transistors, solar cells) which are often probed by EBIC. This amplifier seems better suited to EBIC applications and may be constructed at far lower cost than some commercially available electrometer amplifiers which appear to be widely used.

Descriptions are given of two types of double-tilt top entry goniometers incorporating facilities for varying the specimen level. Both versions of specimen holder are for use in JEOL 100CX and 200CX electron microscopes fitted with the high resolution asymmetric objective lens polepieces of C_s=0.7 mm (at 100 kV) and a top entry stage. The principles governing the design of such holders are discussed, as are the relative advantages of the two holders, both of which have been constructed and tested. Both holders allow up to +or-15 degrees tilt and do not limit the image resolution attainable by the microscope.

A new, SQUID-based, measuring system has been developed for the investigation of the dynamic properties of magnetic materials. Its main advantages, compared to conventional mutual inductance systems, are its high sensitivity and its nearly frequency independent response, down to extremely low frequencies. With the SQUID system it is now possible to measure directly and rapidly the frequency dependence of the dynamic susceptibility of weakly magnetic materials in the range from 0.002 Hz to 5 kHz at a fixed value of a constant background field of up to 5 T. The frequency-swept operation allows experiments which are beyond the capabilities of conventional systems, for example the investigation of thermally isolated samples or the study of the dynamics at magnetic phase transitions in cases where the magnetic properties change very rapidly in a small field interval.

For moderate requirements of field uniformity, about +or-2% referred to the centre of a cube of (0.4 m)³, an active shielding system consisting of a single, rectangular, folded loop is proposed. The loop can simultaneously reduce the horizontal and vertical components of the external field down to a level determined by the remaining non-uniformity. The length of the horizontal and vertical legs of the loop must be designed to fit the local field of the experimental region. Some modifications are possible so that a smaller second horizontal component can also be accommodated. The field uniformity of the folded loop compares favourably with that of other similar single or two-loop coil systems of cubic shape.

A torque transducer with improved accuracy at low torque has been developed for a passenger car drive shaft application. The device is described and its design and development discussed. A major feature of the device is the exploitation of a nonlinear torque-strain characteristic to improve the measuring accuracy for small torques. The nonlinear characteristic is shown to impose some novel signal processing problems which have been solved. The signal is digitised within the transducer to overcome the deficiencies associated with conventional slip-ring systems. Test data for the device is presented.

Triple hot-wire sensors used in highly turbulent flows with unknown flow direction exhibit significant errors due to their limited response to yaw and pitch angles of the instantaneous velocity vector imposed by their individual geometrical characteristics. A stochastic error analysis has been attempted which quantifies the errors arising from these reasons as functions of the turbulence intensity for various hot-wire configurations. The analysis shows that even in the case where the mean flow direction is not known beforehand, mean velocities and turbulence intensities can be measured quite accurately even at a turbulence intensity of 30%. These results agree fairly well with existing experimental data.