Table of contents

This is the only measuring technique developed so far which allows determination of the flow velocity directly, with high spatial and temporal resolution and without any disturbance of the flow itself. The behaviour of particles which act as flow velocity indicators is discussed, and the principles of three different types of laser anemometers are outlined. Their advantages and disadvantages with respect to accuracy of the measurement result, spatial resolution and optical stability and their applications to high speed flows are described in detail.

For pt.II, see ibid., vol.8, p.161 (1975). Speckle noise arises when coherent radiation of any kind is used in an instrument. It can be both subjective (formed in the detector) or objective (in the beam itself). A theoretical description of speckle noise given here leads to a prescription for the elimination of both subjective and objective speckle in an important practical case-the diffusely illuminated hologram.

A modification to the Hersch cell is described which permits the measurement of oxygen concentrations in the gas phase of encapsulated electrical switches. Using this technique 1 ppm of oxygen can be detected in a reed relay unit of volume 0.1 cm³.

A pneumatic circuit has been developed to actuate repetitively any switching element lending itself to a reciprocating motion at a cycle period variable typically from 75-400 s. The device features operation free of electromagnetic transients. It is relatively inexpensive to fabricate and easy to operate.

A reliable and inexpensive circuit is described which allows the peak voltage produced by a Gen-Tec pulsed laser energy detector to be accurately held, allowing energy measurements to be conveniently made by readings from a moving coil meter. The circuit has excellent linearity and a low voltage drift in the hold position, and is suitable for voltage peak hold applications in general.

Cold pressure welding has been used to produce a lap junction between 0.15 mm thick strips of copper and constantan which had been plated with 2 mu m of nickel. The junction thickness after welding was 0.10 mm, and after further cold rolling to 0.06 mm a thermocouple performance within the requirements of BS 1041 (1966) was obtained.

A twenty stage electron multiplier using aluminium as dynode material is described. When operated in DC mode, very stable gains approaching 10⁶ were obtained with input currents of the order of 10^-12 A, even after repeated exposures to the atmospheres.

The principles of operation of an electron microscope specimen tilt stage are described. A stage using these principles has been constructed and permits tilt to an angle of 17.5 degrees about any chosen axis in the specimen plane.

A modification to the 51F32 low pressure transducer was devised to convert it into a 0.5 in internal diaphragm transducer for use in pressure scanning switches while still retaining the flush diaphragm capability. Fully dimensioned working drawings and calibrations of frequency and phase response for a range of tubing lengths are given.

A simple low cost polarizer and analyser for thermal neutrons is described which allows a 97% degree of polarization irrespective of the neutron wavelength. The necessary magnetization is achieved by commercially available permanent magnets directly attached at the rear of the Vacoflux 50 mirror sheets.

The gelation time of a gelling solution can be determined using viscosity-time data obtained at constant temperature. An instrument has been developed which enables the gelation time of a solution gelling to be measured at constant temperature. The viscosity is measured in terms of the free fall velocity of spherical drops of carbon tetrachloride through the gelling solution which is measured electronically using a fibre optics detector system. The quantity recorded and displayed digitally by the instrument is the fall time, or the time taken for a drop to fall between two fixed points. The gelation time, t_g, can be calculated using an empirical relation which accurately describes the solution viscosity as a function of time up to the point of gelation.

The characteristics are investigated by calculation of the energy distribution of the electrons whose trajectories pass through the spectrometer. This work is an extension of previous calculations on the trochoidal electron monochromator on which the backscattering spectrometer is based. A comparison between certain of the predicted characteristics and those determined by experiment is made.

Undistorted waveforms from sequentially activated sensing devices can be registered on a single wavetrain by delayed blocking of each output signal. A scaling method and fast sweep technique allow the results of a large number of measuring stations to be recorded on the screen of one dual beam oscilloscope with no loss of information. The total display time can be as much as ten times the duration of a single, cross screen sweep, depending on the amplitude of the scaled signals.

It is shown that, in the absence of saturation, this method, which can be applied for arbitrary line shape functions, provides the true line shape as well as any of its derivatives. The same holds if saturation is present, provided that the modulation frequency is small compared with the spin-lattice relaxation rate and the induced transition probability. The influence of the correction procedure on noise is also investigated.

A very quick, reliable and reproducible procedure for preparing 'holey' films with various ranges of hole diameters (0.05-20 mu m), as test objects for correction of astigmatism in high resolution instruments, and Formvar nets for mounting ultrathin microtome sections for high resolution electron microscopy is presented. It has been possible to control the hole diameter within limits under the stated conditions. The physical explanation for the mechanism of hole formation has also been given.

An ultracentrifuge used to magnetically support and spin small rotors to high rotational speeds is described. Electronic circuits for the magnetic support system and the rotor drive are given. The highest speed attained with the equipment was 17500 rev s^-1.

Experiments have been carried out to investigate, by means of picosecond mode locked laser pulses, the performance of the fast vacuum photoelectric cells associated with a high sensitivity, high speed cathode ray tube provided with a channel plate electron multiplier. An overall time response of less than 100 ps has been measured with an amplitude sensitivity of 140 mV cm^-1.

The principle of measurement employed has the combined merits of the 'null detection method' and the 'direct reading method'. The instrument is simply operated to give high precision data over a wide range of frequency from 3.5-110 Hz and a temperature range from liquid nitrogen temperature to about 250 degrees C. Activation energies and the complex piezoelectric strain constant can be also evaluated.

An alternating field (AF) demagnetizer capable of generating a peak field of 0.1 T at a frequency of about 300 Hz, with provision for tumbling the sample about two axes is described. An optoelectronic device, driven by linear and digital integrated circuitry, is used for transient-free closed loop control of the field. The entire sequence of a demagnetizing cycle (the rise of the field to its exact preset peak value and its decay to zero) is digitally controlled. The amplitude-time profile of the field is well suited for the demagnetization of rock samples.

A control circuit is described which maintains a constant incident beam current for LEED-Auger electron guns. Significant in the design are isolation of control circuitry and gun by means of a solid state optical coupler, control based on true beam current, and adaptability of circuit to existing equipment. Control to better than 0.5% is possible for beam currents between 0.1-100 mu A.

The method avoids the undesirable effects arising from the constraints on a capillary specimen, whilst allowing the specimens to be used many times without deterioration, unlike bare alkali metal wires.

A DC excited thermistor bridge, and a thyristor controller have been used to provide enhanced temperature stability in a 10 m long strainmeter testbase. Short term temperature variations are held within 40 mu K standard deviation. Drift over 800 h is less than 130 mu K h^-1 indicating that thermistor sensors can provide extreme stability in fixed point applications. The relative bimetallic length change occurring between the steel base, and the monitoring vitreous silica catenary strainmeter has been maintained at a stability varying between approximately 10^-10 to approximately 10^-12/h, being limited by ambient influences rather than by testbase temperature changes.

The problems involved in devising a measurement method are discussed and some practical solutions are suggested. The technique has been applied to liquid Hg and liquid Li, where it yields data in good agreement with conventional methods, and has been extended to cover a variety of alkali-metal based liquid alloys.

The high and low impedance switching pulses can be triggered up to a repetition rate of 1 kHz, synchronously sampling the resistivity of the specimen after each switching pulse. If a failure in one switching direction is observed, the subsequent reverse switching pulse is suppressed and pulses for the failure direction only are applied (multitry technique) until switching is achieved, whereupon the normal alternating set and reset operation is automatically reinitiated. An automatic interrupt is suggested as a supplement to discontinue the test procedure after 16 successive switching failures. With this arrangement, the reliability of SiO films sandwiched between a Co-Ag electrode pair was investigated.

The very large (>or=15 cm) out of focus electron microscope techniques used for detecting electric and magnetic fields producing angular deflections of 10^-5-10^-6 rad are considered. Observations made on weak Lorentz objects, such as the electric field of p-n junctions, (i) by high voltage transmission electron microscopy (TEM) and by high voltage scanning transmission electron microscopy (STEM) performed by means of an attachment to a TEM instrument; (ii) by low and high voltage TEM are compared. The results show that high voltage TEM is very satisfactory-better than conventional TEM and far superior to STEM in imaging very weak Lorentz (phase) objects even in thick specimens, at least with the present state of development of STEM attachments. The operational conditions are given.

A description is given of a gas driven circulation pump for liquids which are unstable if they contact material other than glass or a gas. The performance of the pump is independent of temperature. The maximum flow rate does not depend on the resistance of the circulation system and can therefore by attained at all differential pressures between the entrance and exit of the pump if the pressure of the driving gas is raised sufficiently.

Use of the techniques of time domain spectroscopy (TDS) to determine the microwave properties of materials is a rapidly developing field. Two recent advances in techniques are described. Firstly, a method for reducing the uncertainty with which the time domain waveform can be referred to a common time origin which permits 'time referencing' to better than 0.08 ps-more than an order of magnitude improvement on the use of more conventional 'time markers'-and requires only the basic TDS instrumentation, and secondly, a variation in TDS technique involving the use of a high pass filter. By eliminating the effects of 'low frequency noise', a better defined pulse shape for Fourier transformation is produced, thus reducing truncation errors.

A cheap, simple recorder designed to operate unattended for long periods from small batteries is described. Signals are recorded on a magnetic tape cassette driven by either the hour or the minute hand shaft of a battery clock movement and may be played back into a computer. Two continuous signals can be recorded and reproduced with an error of 2% of the signal range, while events can be recorded with a time resolution of a few seconds.

An optical magnetometer is described for measuring K_u (the uniaxial anisotropy constant) and K₁ (the first magnetocrystalline anisotropy constant) from about -40 degrees C to the Curie temperature ( approximately 120 degrees C).