Table of contents

A computer-linked microdensitometer has been constructed by fitting a stage with cross-slides driven by impulse motors to a commercial projection microscope. Its accuracy and speed are compared with those of a cathode-ray-tube microdensitometer. The procedure and requirements for analysing x-ray photographs from biological fibres are described.

An electron-nuclear double resonance spectrometer to study the Overhauser effect at magnetic fields near 30 G and over a wide temperature range is described. A combination of high Q in the electron resonance cavity with sensitive nuclear resonance detection makes observations on high dielectric loss samples practical. Examples of enhanced nuclear resonance sensitivity (for protons, fluorine, lithium and phosphorus nuclei) are given with an application of the apparatus for very accurate measurements of electron spin resonance parameters at low fields.

A technique is described for recording the electron spin resonance spectra of paramagnetic species existing at different stages of a photochemical reaction. The sample is placed in the cavity of an electron spin resonance spectrometer and irradiated with 100 μs flashes of ultra-violet light, at the rate of one flash per 1·5 seconds. The presentation is obtained by six parallel sampling channels that sample the output from the spectrometer at different intervals after each flash. The field of the magnet is swept slowly and the output from each sampling channel describes a different spectrum. As an example of the use of the spectrometer results are presented for the flash-photolysis of benzophenone in an isopropanol and water solution. The sensitivity is compared with that of some other techniques.

Outputs from fifty thermocouples are recorded by converting the movement of a galvanometer light spot into a series of electrical impulses which are then amplified, shaped, counted, and finally printed on a paper strip. The thermocouples act as wet- and dry-bulb thermometers to measure the relative humidity of vapour in equilibrium with small samples of soil or plant material and the principle of Peltier cooling is exploited to deposit water on the wet bulbs. Passage of the cooling current, switching of the thermocouple circuits and the initiation of counting and printing operations are controlled by a programme timer and a selector which allows any combination of psychrometer units to be scanned repetitively. The precision of the system (equivalent to one digit) is ±0·0007 degC or 0·007% in relative humidity at 25°C or 10 J kg⁻¹ (1 metre) in water potential.

Electrometer tetrodes lend themselves to use in a balanced bridge-type circuit, notably that due to Penick, in which the output meter indications are made sensibly independent of supply voltage over a limited range of the latter. Producing a working circuit has, however, been largely a matter of trial and error. By contrast, the new circuit is amenable to a straightforward analytical design procedure, and can provide the basis of a very simple high input-impedance null detector having an excellent performance and a wide range of applications.

A current- or voltage-to-frequency converter is described for converting analogue signals into a pulse form in which the pulse repetition frequency is proportional to the instantaneous signal amplitude. The conversion is linear to within 0·01% over the frequency range 0-10 kHz (or 0-1 kHz), and to within 0·1% up to 50 kHz. Thermal stability is comparable.

The circuit action is based on continuous integration of the input signal, and the converter may be used as an interface in signal processing, or as an integrator, or for converting a digital frequency meter into a digital voltmeter with 10 μV resolution. It may also be used in reverse as an accurate count-rate meter (frequency-to-voltage conversion).

A concentric, guarded probe assembly suitable for measurements of the distribution of static electric charge density on dielectric surfaces is described. The procedure for calibrating the probe is given. Application of the device is illustrated by studies of the density of electric charge on a perspex surface.

An atmospheric spark gap is described which is capable of operating in circuits up to 40 kV, with peak currents of 250 kA and passing in excess of 250 C per discharge. The switch, which has an inductance of 100 nH, was designed and tested mainly as a clamp switch with an arc drop of 100 V, but will also operate as a start or divert switch. The arc behaviour is described for times up to 5 ms.

A simple and rapid technique for the determination of the equilibrium vapour pressures above volatile melts is described. Based on liquid encapsulation, the technique should be applicable to most metal and semiconductor melts. The thermal analysis of volatile melts may also be done by the same technique and even at the same time.

A pump which delivers from 0·01 to 0·65 ml fluid per beat at a frequency of 10-80 beats per min is described. Consecutive discharges agree within 1·5% of the average. Pumping is achieved by rhythmically compressing an elastic tube which has one-way valves at either end. Compression is achieved by oil pressure and controlled by an electronic regulator. This pump surpasses other micropumps with regard to precision, lack of hemolysis and versatility of capacity, frequency and pressure profile.

This paper describes a simple multichannel manometer for aerodynamic and gas-dynamic measurements. The instrument makes use of optical interferometric techniques, using a laser light source. The output of the manometer is in the form of a photographic record of an interference fringe pattern, from which variations of pressure with time may be deduced. The sensitivity of the manometer remains constant over a wide pressure range. The response of the instrument is considerably faster than that of a liquid manometer or mechanical pressure gauge, and approaches that of a conventional pressure transducer. The main factors limiting the response are discussed.

The transformation of an electro-optic modulator for use with a helium-neon laser from a research model to a production item is described together with detailed requirements for temperature stability and parallelism of the crystal faces. It is shown that the lengths of the crystal pairs have to be identical to ensure temperature stability and that a total wedge angle of 10 seconds of arc is acceptable. In the course of the work, it has been possible to redetermine the value of the transverse electro-optic coefficient for ADP, which is given as r₄₁=24·7±0·3 pm v⁻¹.

The insertion of a skew Brewster angle attenuator into a laser cavity results not only in a loss, but also in the rotation of the plane of polarization of the emitted light. Equations which govern these effects are given and the result of experiments performed with a helium-neon laser to verify these equations are described.

Equipment suitable for general laboratory use for the cleaving and string-saw cutting of crystals during the preparation of experimental specimens from virgin ingots is described.

A simple device has been developed for stopping a geared motor in a preselected quadrant in situations where overrun must be avoided.

An adaptation is described of standard remote-handling tongs to enable manipulations to be performed in a high vacuum or high purity environment. A variety of operations is possible using commercially available tong attachments or by modifications to an existing attachment. The tong assembly described has been used successfully in a vacuum of better than 10 μtorr over the full range of 15 inch axial movement and 40° solid angle deflection. If necessary, the maximum axial and angular movements can be increased by appropriate design modifications.

A simple method is described for the metering, control and mixing of gas flows to provide gas atmospheres, or mixtures, of stable composition containing very low concentrations of one of more species.

A servomechanism circuit has been developed for use with Helmholtz coils, to compensate for random magnetic field fluctuations of the order 0·5-20 mG or greater having a period of more than 0·05 S.

An improved design of laser output meter is described, in which the detector, which acts as a black body, is in the form of a double cone, constructed from a single layer of anodized aluminium wire. Energies in the range 20 J-1 mJ can be measured without the use of attenuators. The device is simple to construct and calibrate.

A technique has been developed for simultaneous irradiation of 10 μl liquid samples in quartz capillaries for photochemical measurements. The method, in which the volatile samples once they are outgassed are kept from the atmosphere until analysis, is shown to be quantitative, with an accuracy to better than ±3% from sample to sample and batch to batch. A method for nonvolatile liquid samples is also described. Methyl propyl ketone has been used as a satisfactory chemical actinometer at 3130 Å in the capillary system. The quantum yield of formation of acetone from methyl-n-propyl ketone is 0·123 and from methyl-n-amyl ketone is 0·231 in the pure liquid at 25°C at 3130 Å.

Tests of sintered pressed disks of potassium bromide have shown that they are satisfactory window materials for the measurement of infra-red spectra at pressures of up to 300 atm and temperatures of 50°C.

An effective technique is described for high-power pulse excitation of any practical length of a cylindrical laser medium.

A method is described of constructing a homogeneous magnetic field with the aid of air-cored coils of semi-ellipsoidal geometry, for application in nuclear spectroscopy.

A simple straining device has been developed for use with the Cambridge Instrument Company's Stereoscan scanning electron microscope. The device replaces the normal specimen holder and no modifications to the stage are required. The effect of stress on the surface topography of fibres and other specimens may be observed directly on the display screen.

Velocity measurements in thin oscillatory boundary layers require an instrument with high natural frequency and sensitivity, and small bulk at the point of measurement. An instrument which possesses these qualities has been constructed for use in two-dimensional flows. It consists essentially of a thin fibre suspended at right angles to the stream. As the velocity of the stream varies so does the tension in the fibre. Thus, measurement of the tension allows the velocity to be determined. The results obtained with this device have been very satisfactory.

A technique is described which employs the usual method for preparing surface foils for transmission electron microscopy, but with one additional stage. Prior to thinning, the deformed surface is shadowed with a heavy metal in the manner normally used for replicas. The improved contrast given by this shadowing facilitates the direct correlation of surface deformation markings and the underlying dislocation structure.

Zinc sulphide antireflection coatings of quarter-wavelength thickness are applied to ferromagnetic surfaces to improve the visibility of ferromagnetic domains viewed by use of the Kerr magneto-optic effect. Simultaneous monitoring of destructive interference effects at three wavelengths, in light reflected from films during deposition, allows coating to be automatically terminated at, for instance, a quarter-wavelength thickness for a selected mercury line.

An airtight cover has been designed for use in a Cambridge Stereoscan scanning electron microscope to allow examination of hygroscopic and easily oxidized substances which must not come in contact with the air.

A simple vibrating contact arc ion source is described that will produce currents of heavy metallic ions in the microamp region. Results of mass analysis are given showing a significant proportion of multiply charged and molecular ions. It is concluded that cathodic sputtering is largely responsible for the maintenance of the arc, especially when metals with high melting point and low vapour pressures are used.

This note describes an inexpensive, two-transistor, 100 kHz amplifier capable of driving a one-turn modulation loop inside a TE₀₁₂ sample cavity of an X band electron spin resonance spectrometer. Modulation fields up to 8 G peak-to-peak can be obtained.