Table of contents

A general account is given of both the electrical and the mechanical aspects of the design of capacitative transducers and their associated electronic circuitry suitable for observing displacements of the order 10^-2 to 10^-11 mm. The lower figure is the order of magnitude of noise and drift averaged over a second, the drift over a day of the order 10^-8 to 10^-9 mm. Their application is illustrated by descriptions of an apparatus to explore the limits of performance, a moderately sensitive micrometer, and two geophysical instruments, a tiltmeter and a gravimeter. Full details of a general purpose electronic system are given.

It is shown that the collector-coupled astable multivibrator whose one base resistor is replaced by a thermistor represents a temperature-to-frequency converter whose characteristic can be linearized in the given temperature range. The linear coefficient of the characteristic of the practical converter in the range 20-50 degrees C is beta =2.4% degrees C^-1, with linearity to within +or-0.25 degrees C and dependence on the ambient temperature of less than 0.027% degrees C^-1.

Describes a simple method for securing the end windows of a liquid laser cell using adhesives. The seal has proved resistant to ethanol, methanol and liquid host materials for the Nd³⁺ ion.

A design of an osmotic membrane is described for continuously monitoring hydrogen partial pressure in experiments conducted within internally heated gas pressure vessels. The advantages of this design include faster diffusion rates through the membrane (hence faster response), ease of construction and adaptability to high temperature apparatus.

A laser excited Fabry-Perot interferometer for measuring electron densities (N_el>5*10¹⁵ cm^-2) is described; N_e is recorded directly as a function of time. The plasma is produced in a z-pinch discharge (helium gas) and is located in the space (length l) between two tubes which define the resonator axis. The dependence of N_e on l shows the tubes do not contaminate the plasma, and measurements at two wavelengths confirm that the plasma refractivity is produced by free electrons. The resolving time for the density records is of the order of 50 ns.

An instrument has been developed which will measure a rapidly changing capacitance of 0.3 to 0.7 pF to an accuracy of +or-0.002 pF. The instrument faithfully followed square wave input using an ultraviolet recording galvanometer of linear frequency response up to 100 Hz. It has been used for measuring the growth rates of bubbles in the size range 0.1-8 cm³ injected into particulate gas fluidized beds.

A means of making satisfactory connections to thermocouple leads within a high pressure vessel is described. This device enables the leads which are situated in the transition zone from high pressure to atmospheric to be maintained at the reference temperature of the thermocouple. The connecting device is made from readily available high pressure accessories.

A description is given of the design of a variable temperature cryostat with a tail section which is elliptical in cross section. This somewhat unusual configuration achieves a narrow cross section in one dimension, typically for use between the poles of a magnet, without the problems normally associated with conventional small diameter cylindrical designs.

A laser interferometer for the accurate measurement of displacements is described. The use of an XY oscilloscope leads to a simple design and an easy adjustment of the interferometer and improves the resolution of the system to about 4 nm.

The design and performance of an automatic, four-terminal resistance bridge for low temperature resistance thermometry are described. An overall accuracy of 0.5% and a resolution of 1 part in 1000 are achieved with a 10^-12 W sensor dissipation. Continuous analogue balancing is employed to yield a maximum response time of 7 s for a step change of one decade in the resistance. Experiments on three different developmental self-balancing bridges are also discussed.

An apparatus to measure the dispersion characteristics of thin film capacitors at extremely low frequencies, even below 0.01 Hz, is described. This makes it possible to study the slow motion of mobile ions in the dielectric. The apparatus runs completely automatically.

A double AC method is described for measuring the Hall voltage of thin semiconductor samples in the temperature range 4-300 K. The magnetic field is produced inside the cryostat by means of a built-in electromagnet. By this new arrangement fields up to 1 Wb m^-2 are achieved at field frequencies of 2 kHz or less. Hall voltages of some 10 nV have been measured. The upper limit of sample resistance is at 10 G Omega . The smallest detectable mobility is of the order of 10^-4 cm² V^-1 s^-1. Comments are given concerning the proper selection of current frequency and magnetic field frequency. Results of mobility and carrier concentration for a high and a low resistivity sample are given as a function of temperature.

A new technology for producing large volume microwave plasmas is introduced, Microwave energy is coupled into the plasma by means of a slow wave structure, and plasma volumes well in excess of 1000 cm³ can readily be achieved with 2.5 kW of microwave power at 2.45 GHz. General operating characteristics and some experimental data are presented.

Describes a temperature controller for magnetization or susceptibility measurements at temperatures between 4.2 and 7.5 K. Methods are given to allow an easy construction of the controller and to avoid thermal oscillations. A stability better than 1 mK is obtained over a period of one hour. The response time for an increment of 1 K is about one minute.

An optical reflectance attachment designed to measure the reflectance properties of leaves and grain seeds, ground or whole in the spectrum 0.4-32, is discussed. Front surface mirrors are used to cover the spectrum, and depending on the wavelength band chosen, different detectors and radiant energy sources are used.

A small, inexpensive device for determining the spin rate of a sounding rocket is described, which employs the variation in resistance of a magnetically sensitive diode when it is rotated in the earth's magnetic field.

Describes an apparatus for the measurement of the optical absorption of aqueous samples, in the wavelength range 200-800 nm, at pressures and temperatures up to 100 MP and 900 K during gamma irradiation from a ⁶⁰Co source. The application to radiation chemistry studies on the primary products of radical-scavenger reactions is described and an extension to the measurement of steady state free radical concentrations suggested.

A fast multiscaler is described which is based on a PDP 11 computer. The actual multiscaling is done by the interface using the PDP 11 as an easily accessible core memory. The shortest channel time is 2.5 mu s, the pulse pair resolution is 80 ns and the dead time between channels is 50 ns. The number of channels and the capacity per channel are only limited by the available core memory.

Frequency separations have been measured of some of the ¹²⁷I₂ saturated absorption features used to stabilize the 633 nm He-Ne laser. The frequencies are measured with uncertainties approaching 10 kHz by a digital beat frequency method, using lasers locked to the absorption features by a third harmonic technique and having stabilities of 10 kHz ( delta nu / nu =2*10^-11) for a 10 second observation time. Wavelength measurements are made on a laser locked to the 'd' absorption feature, using a krypton-86 lamp in conjunction with a scanning Fabry-Perot interferometer. The value lambda _vac is 632991178.3 fm with a statistical uncertainty of +or-0.8 fm ( delta lambda / lambda =+or-1.2*10^-9).

Magnetic fields in the range 100-200 T are generated in lightweight coils with an inner diameter between 2.5 and 10 mm. The rise time to peak field is 2 mu s. Special design features of the 20 kV, 55 kJ capacitor bank facilitate the application to experiments. Thin-walled coils made of dense, ductile material give superior performance in this field range.

The source of the continuum in the BRV pulsed light source is investigated. Electrical and optical measurements show that the continuum is radiated from a fast pinch discharge X-ray measurements give a temperature of 5 keV. Spectra of the far UV continuum are shown and the intensity is measured. The optimum capacitor size is established.

In a double-coil magnetic comparator of the oscillating type certain systematic errors and limited sensitivity will exist due to the finite value of the amplitude of oscillation and to misalignment of the directions of the two magnetic fields to be compared. These effects are analysed, and it is concluded that even with a simple null detector the accuracy of the comparison may be of the order of 10^-2% if reasonable care is taken regarding the alignment of the fields.

Details are given of an electron spectrometer possessing essentially constant transmission characteristics in the energy range up to 200 eV. As examples of its application, results are presented for resonant differential elastic scattering in He and for the (2s²2p) ²P and (2s2p²) ²D resonances in the 2¹S and 2¹P inelastic channels, also in He.

The design, construction and three typical applications of a versatile interface for PDP-8 computers, adaptable to PDP-11 systems, are described. The interface is conceived for on-line control and data acquisition in a wide variety of physical and chemical experiments. Design principles and specifications as developed from experience are discussed. A detailed description of such a system is presented, together with three typical on-line applications in the field of ESR, ENDOR and Fourier spectroscopy.

Two methods are commonly used for collecting rock samples suitable for palaeomagnetic study: (i) block or hand sampling; (ii) drilling. To facilitate field orientation of samples collected by the block method, a lightweight pocket-sized instrument has been designed. The compass-clinometer consists of a 4¹/₈ in*4³/₄ in *¹/₂ in perspex table which incorporates a 4"-360 degrees fixed sum compass scale; when not in use, brass rods for the sun compass are conveniently housed in a slot in the table. An inner rotating perspex disc contains the magnetic compass and the clinometer. A variety of orienting conventions are easily accommodated.

In order to avoid damage to optical components one can use masks at appropriate places in the system or antireflection coatings for the curved surfaces. Another simple solution to the problem is to rotate a lens, for example, to a slightly off-axis position or to vary its location, if possible.

Deals with the deconvolution of the response by fast Fourier transform and matrix inversion techniques.

The authors criticise the method of Roberts (see abstr. A43553 of 1972) and present another similar technique.