Table of contents

A discussion is given of the physical and mathematical properties of noise together with the concepts of noise bandwidth and system bandwidth. A general design procedure for measurement system is then outlined and illustrated by reference to a magnetometer developed by the author.

During the operation of a high power pulsed CO₂ laser system, damage was observed to a GaAs modulator crystal. The cause of the damage was possibly the creation of wedge fringes within the crystal. Wedge fringes were also formed in the system by a Ge polarizing plate. The authors describe how the formation of such fringes can be avoided and also how they can be used to measure small wedge angles over long path differences.

An orifice assembly for particle sampling in, for example, mass spectrometry is described. It consists of a holder and an insert which clamp a microhole disc into position in the wall of a system from which particles are to be sampled. Microhole discs with hole diameters of 1 pm and greater are available. Compression of the disc provides an adequate vacuum seal without distorting the microhole.

A method of making a tungsten electrode for an arc caster without the need of hydrogen-brazing tungsten to copper is described. The electrode so made performs as well as the one made with hydrogen brazing.

A technique is described for detecting dynamite and trinitro-toluene (TINT) with high specificity, by means of an electron capture device. Electronegative vapours from these explosives can easily be detected by electron capture at room temperature but decompose at 250°C and therefore cease to be detectable. However, electronegative vapours from nonexplosive substances remain stable and detectable at this temperature. An instrument which makes use of this difference and is thus specific for explosives has been developed ; it uses commercially available electron capture sensors.

A time measuring unit is described which is useful for incorporating in portable laser range finders. The unit possesses some special features which are essential for range finding applications. These include, among others, minimum range gating and multiple target discrimination.

Difficulty in obtaining very low noise difference amplification with gain stability using conventional operational amplifier circuitry is considered. A design procedure eliminating the incompatibility of gain stability and low noise is discussed. The design, circuit details and performance of low noise, high input impedance amplifier with good gain stability and wide offset range is described.

A current pulse generator is described which has been developed for use in connection with a pulsed nuclear magnetic resonance (NMR) spectrometer. It is capable of producing current regulated pulses in the range 30-120 A at pulse widths of 0.1-10 ms. Twin pulse separations of 0.1 ms at repetition frequencies of 3 Hz have been achieved. Typical rise times into resistive loads are of the order of mu s( approximately 80%) for a 120 A pulse.

Cathodoluminescence collection could be improved by using an ellipsoid mirror with a photomultiplier installed directly in the second focus of the mirror. No losses of light due to the light guide coupling can occur in this way.

A dye cell has been designed for use in high power laser-pumped dye lasers. It combines some of the advantages of longitudinal and transverse pumping and avoids their drawbacks. The gain profile is very smooth, and power density and mode purity can be chosen according to the users' requirements.

A design is described for a new type of radiometer for monitoring flame radiation in fire extinction experiments. The radiometer uses a photocell and eliminates many of the practical difficulties encountered with the conventional thermocouple type of radiometer currently in use.

An accurate and robust flowmeter with no wearing components and a voltage output suitable for use with a potentiometric recorder, can be constructed around a mercury in glass manometer modified into a three electrode differential capacitance bridge.

Describes the basic theory and design of a pressure control system for the Pepsios spectrometer. The proposed pressure control system utilizes a simple needle valve and is simpler than that used by previous workers. Several alternative designs have been considered and it has been found that the simplest case of a fixed leak to a vacuum is suitable for a Pepsios spectrometer. Hence the equation governing the response of the system for fixed leakage to a vacuum line has been derived. The system response has been obtained by solving the second order differential equation of the pressure control system using an analogue computer. Selection of parameters for optimum design (best response) of the system is subsequently discussed.

To perform mathematical operations on data available as graphic recordings, an apparatus has been developed for the recovery of analogue signals from the recording itself. This device, based on repetitive optical reflection scanning, is capable of simultaneously processing up to eight curves drawn on the same chart. The useful operating area is 27 cm*45 cm, but simple modifications can be introduced to treat strip chart recordings. The electronics used for data conversion are partly digital and use sampling techniques. Special provisions have been introduced to minimize noise problems during the trace detection process. Traces down to 0.5 mm in width are easily detected and the overall accuracy is better than 1 mm.

An apparatus is described to measure internal friction based on the successive measurement of the velocity of a reflected light beam at the beginning of and after a predetermined number of oscillations during free decay. Results obtained using this method are compared with those obtained using the amplitude method and agreement is excellent. The main advantage of the present method is that it works over a wide frequency range (0.3-60 Hz) on a single wire specimen.

An electrostatic liquid filter is described which operates by atomizing the liquid with its impurities in a divergent electric field. The solid impurities are subsequently collected by an ultrasonically cleaned grid. The conductivity and electric strength of transformer oil filtered with this device indicates that it has a performance approaching that of a 1 mu m sintered glass filter, but the device has an increased throughput.

The apparatus described allows the stored charge in a diode from the reverse recovery behaviour to be measured. It permits an adjustable time delay between the end of the forward current pulse and the application of reverse bias. The amount of charge recombined during the delay interval may be evaluated easily and a lifetime determined. For convenience, a digital integrator and programmer has been developed, allowing an easy and accurate valuation of the stored charge.

A high resolution circular dichroism spectrometer has been constructed primarily to observe low temperature, magnetic circular dichroism (MCD) spectra of crystals. The design incorporates two novel features: simultaneous measurement of the absorption spectrum and compensation for the dark current of the photomultiplier. Examples of the performance are shown both for crystals and for a solution.

Two Zeeman modulation devices are described. In the first one, the magnetic field in a solenoid is square wave modulated at 1 kHz. The second one uses two orthogonal sets of Helmholtz coils. The direction of the magnetic field is sine modulated at 12.5 kHz and its amplitude may be chosen in the 0-10 G range. The resolution and the sensitivity of the apparatus made it possible to measure new Lambda doubling transitions of OH. The Zeeman splitting in the earth magnetic field has also been observed on the SO radical.

Describes a system for improving the sensitivity and noise rejection of millimetre and submillimetre wave transmission and reflection spectrometers. The performance is compared with conventional methods for the same measurements. The system was designed primarily for use in a solid state magneto-plasma transmission experiment requiring intense, pulsed magnetic fields, but can be used in other pulsed and steady state experiments, for example, pulsed antiferromagnetic resonance, magneto-optical studies in semiconductors, magneto-phonon effects. An analysis of the performance of microwave diode modulators is also given.

A measurement technique is described by which the laser induced surface temperature rise in metals can be determined experimentally with laser pulses as short as 10^-6-10^-8s and intensities of up to 10⁶-10⁷ W cm^-2.

Previous calculations of the trajectory of ions in a field ion microscope have been extended to the situation where the electric field experienced by an ion alters during its time of flight from emitter to screen under the application of a positive voltage pulse applied to the emitter. The calculations indicate that ion trajectories depend both upon the shape and duration of the pulse, and may lead to errors in aiming in the atom-probe mass spectrometer.

A new design criterion for astatic magnetometers with negative feedback is described. A simplified mathematical analysis of the dynamic unbalance of the magnet system which give rise to noise is presented along with its experimental verification. For a given time of observation two techniques for enhancing the signal-noise ratio are outlined and demonstrated by consistent measurements of magnetic moments of rock samples in the range 10^-3-10^-9 A m².

Design criteria are discussed and circuits are presented for a phase sensitive detector and its reference signal generator to be used in third derivative locking of the frequency of a laser to the centre of a saturated absorption feature. The use of a phase locked loop enables the operating frequency to be continuously variable over the range 4.7-47 kHz so that it can be matched to a mechanical resonance of the transducer used to frequency modulate the laser.

A column instrument developed as a potential device for the accurate measurement of several thermal properties such as the electrical resistivity and hemispherical total emittance of metal wires and the thermal conductivity of gases over an extended temperature range is described. The results are reported for platinum and neon covering the maximum temperature range of 273-1650K. In each case the results of the present work are found to be in good agreement with the available data in the literature. This is regarded as a conclusive evidence in favour of the column apparatus as a multiproperty measuring instrument.

A novel thermal bistable circuit which consists of a positive temperature coefficient (PTC) thermistor and a constant current generator has two stable states with low and high output voltage level. When the temperature or velocity of a surrounding fluid exceeds a critical value, the circuit is triggered and it rapidly changes its stable state. Both triggering levels of an input thermal quantity can be continually adjusted by circuit parameters. The thermal parameters of the fluid (for instance temperature, thermal conductivity or velocity) and a defined duration of power supply interruption may be detected and memorized.

The design of a simple device employing a water cooled boat surrounded by a quartz tube for growing single crystals of Dy is described. The cooled crucible avoids difficulties due to the high reactivity of the rare earth and the processes of melting, zone refining, crystal growth and recrystallisation can be carried out without any handling of the material between the different steps.