Table of contents

This year, 1968, is the 50th anniversary of the formation of the Institute of Physics. (The Institute of Physics and The Physical Society was formed from the amalgamation of the two bodies.) In order to celebrate this Jubilee we are publishing a series of articles concerning various aspects of instrumentation over the last 50 years.

Liquid semiconductors are considered for use as temperature measuring elements in nuclear reactors. Several possible compositions in the system As-S-Se-Te-Ge have been studied. They are liquid above about 300°C, have temperature coefficients of resistivity of 1 to 5% per degC between 250 and 675°C and show negligible tendencies to devitrification. Irradiation tests have been carried out on both the materials themselves and on experimental thermistors in order to assess their susceptibility to γ-ray induced photoconductivity and fast neutron damage, with encouraging results. Possible temperature mis-indications due to transmutation effects and local gamma heating are shown to be small and calculable.

A procedure is described for calculating from the pulse method data the thermal conductivity of a specimen when its specific heat or density is not known. It is shown that the thermal conductivity K can be calculated in terms of pulse method parameters as K=Qα/LT_M where α is thermal diffusivity, L is sample thickness, T_M is the maximum back surface temperature rise and Q is the amount of heat absorbed per unit front surface area of the specimen. The method and experimental techniques developed to calibrate the test system, i.e. to measure Q as a function of temperature, are presented. A typical calibration curve of Q against T is included with the values of the thermal conductivity of stainless steel 347 which have been calculated from it. The method has been used up to 800°C, and the thermal conductivity values obtained are within ±10% of literature values. The experimental techniques required to extend this method to extremely small samples are also described.

An instrument is described which measures the mechanical shear impedance of liquids in the frequency range 20-100 kHz. It operates through the magnetostrictively induced motion of a thin walled tube. Compared with earlier devices using piezoelectric transducers it is more robust and versatile and the use of tubes allows the sensitivity to be increased by an order of magnitude. The necessary theory is given for the damped torsional motion of an infinitely long homogeneous isotropic tube immersed in a viscoelastic liquid.

An automatic compensated torque balance for the measurement of magnetic anisotropies from helium temperatures upwards in the range of torques 10-10⁴ erg was constructed. The temperature dependence K₁ was measured on the ferrite Cu_0·09Mn_0·61Fe_2·3O₄ and the energy splitting E_t similar, equals 1540 cm⁻¹ of the triplet d of the ion Fe²⁺ caused by the trigonal component of the spinel crystal field was determined.

A piezoelectric scanning Fabry-Pérot interferometer has been developed and is used in synchronism with a multichannel analyser operating in the multiscaler mode such that each channel corresponds to a definite wave number. The system has been used for averaging very weak signals; it could also be adapted for use with a Michelson interferometer (Fourier-transform spectroscopy) or a scanning monochromator system.

The article deals with the essential features of a new electron emission microscope, special attention being paid to the vacuum system and the objective lens. The vacuum of the order of 10 ntorr is attained in the specimen chamber with the help of an ion-getter pump, and the spectrum of residual gases is checked continuously during the operation. The rest of the optical system is evacuated by a conventional vacuum system with automatic valving. A specimen air-lock contributes to quick cycles of operation. Different kinds of emission microscopy are possible with the instrument. An example of the favourable influence of improved vacuum conditions in case of tunnel emission is given at the end.

A lightweight radiosonde is described, together with associated recording and tethered balloon equipment, which has been constructed to enable the effect of the lower troposphere on the propagation of radio waves to be investigated. The instrument incorporates thermistors and radio telemetry, which allow wet- and dry-bulb temperatures to be recorded on the ground, and is therefore referred to as a `psychrosonde'. Temperature errors are less than 0·1°C, so that in steady conditions the radio refractive index of the air may be computed with errors of less than 0·5 N-unit, and the time constant of the instrument is 8 s.

Considerable detail may be obtained of changes in refractive index with height and time, and the technique is therefore of particular use in studying changes of structure of elevated layers, and changes near the ground, both of which have a considerable influence on radio wave propagation. Two examples are discussed, together with some details of the analysis.

A simple photoelectron spectrometer employing a parallel-plate electrostatic energy analyser is described. Preliminary results showing good sensitivity down to a resolution of 25 mv are presented.

A system has been developed for using a wave-length scanning polarimeter to determine the linear polarization parameters of astronomical sources through the visible spectrum, the spectral resolution depending only on the choice of the spectrometer. The spectrum is scanned in steps and at each wavelength setting three analyser positions are chosen and intensity measurements are made by a pulse counting photometer. Effects of intensity scintillation and changes in atomospheric transparency are overcome by a method of chopping and integration. The observational sequence and the final print-out are controlled electronically. Tests on the blue sky, the moon and stars are described.

A colour schlieren system was designed to make instantaneous measurements of the changing methane distribution in a stratified methane-air mixture in a 10 cm wide plastic-walled container. The usual light source was replaced by a grid made of strips of coloured gelatine and the returning light beam was interrupted by a monochrome grid slightly displaced from the focus of the colour grid. The monochrome grid selected the colour to be transmitted, and an image composed of horizontal coloured bands was formed. Displacement of the bands indicated a change in refractive index gradient in the methane-air mixture. Methods of calibration by both calculation and experiment are described.

The paper discusses how the dimensions of an omegatron limit its applications as a mass spectrometer and explains why most omegatrons have a comparable performance in the same magnetic field. An approximate derivation of the shape of the resonance and an explanation of why the trapping voltage must be optimized are also given. Some possible new designs of omegatron are also suggested.

A carrier-frequency bridge method is described whereby rapid changes in electrical resistance can be studied with a time resolution of the order of fraction one-third ms and typical sensitivity of the order of 10 nΩ in a specimen of resistance of the order of 1mΩ.

A sensitive contact potential measuring apparatus based on a novel form of electrostatic driving of the vibrating condenser is described. The apparatus permits contact potential difference measurements between a reference gold electrode and solid catalyst surfaces in vacuum or controlled gaseous atmospheres at different temperatures. The contact potential may be balanced to better than 1 mv.

Two sets of bridge equipment which operate in the frequency ranges 1-17 kHz and 100 kHz-1 MHz, respectively, are described. The main emphasis is on the former for which a new detector was designed. The apparatus was used for measuring the capacitance and conductance of aqueous biological solutions. Several theoretical techniques for calculating the dielectric constant of the solution from these two parameters are discussed together with the relative merits of each method. The results obtained for glycine, diglycine, alanine and urea, none of which had been previously measured at these frequencies, are compared with the extrapolated values from previous work in the microwave region. In no case was an intermediate dispersion region indicated.

By use of a simple chopper-disk and electronic circuitry a respiratory type gasmeter may be modified quite simply and provide permanent records on a chart recorder. Visual presentation of ventilation by means of a moving coil meter and electromagnetic counter is available when paper records are not required. Reset switches allow the instrument to be used for single breath studies.

The paper deals primarily with the elastic stresses in homogeneous (bonded) plane helical solenoids which are inner members of multicoil systems. The terms `bonded' and `unbonded' are defined with regard to solenoid types and conditions under which they operate. Distinction is made between the treatment required for bonded and unbonded solenoids, the work of other authors being clarified in this respect. The case of an infinitely long homogeneous solenoid with non-zero external field is analysed and expressions for the stresses are derived. These formulae are sufficiently manageable to be used in the preliminary design study of real solenoid systems with some loss in accuracy. They are directly applicable under certain conditions for the stress analysis of real solenoids. A set of computed stress curves are included in an appendix.

A three point bend tester suitable for measuring the flexural strength of short lengths of high strength fibres in the diameter range 10-50 μm is described. The theory of bending is discussed and it is shown that good estimates of the maximum strain in a bent fibre are possible for large amounts of bending without an accurate knowledge of the coefficient of friction at the fibre supports. The overall accuracy of the tester operated under appropriate conditions is better than ±5%.

Tests are described on an orifice hot-wire probe suitable for wall pressure fluctuation measurements. The probe was found to respond to the true pressure for both acoustic and turbulent flow excitation, with a useful frequency response up to 5 kHz in air at atmospheric pressure. A traversing pair of probes proved to be accurate for the measurement of wall pressure correlations in lowspeed air flows.

An electromechanical analogue-to-digital conversion unit for strip-chart and circular-chart records is described. The unit, designed specifically for processing hydrometeorologic records, features chart distance-controlled sampling, paper tape output and continuously variable chart transport speed.

The circuit parameters of a resonance coil, as measured with a commerical Q-meter, have been used to determine the amount of reduced metal in a nickel catalyst bed. A coil for in situ use at reaction conditions is described. Calibration curves, temperature variation effects and an illustrative example of a practical use are given.

The apparatus is based on the use of hypodermic needles and small diameter tubes as the primary metering elements. In order to assure reliability of operation three conditions must be satisfied: the flow choked at the tube exit, the friction coefficient maintained strictly laminar, and the gas behaving as a continuum. Construction of the apparatus, with proper consideration of the above conditions and their experimental verification, is presented.

A technique is described for stripping thin (25 Å or less) oxide films from metals for electron microscopy. Plastic is used as a temporary support and a tartan carbon film is retained as a permanent support.

Normal heater circuits used in calorimetry are investigated from the standpoint that any change in the resistance of the circuit components should not change the power dissipated in the heater. Only a `general' circuit satisfies the required conditions and the choice of resistances for this circuit is discussed.

A new particle size analyser is reported. It is used principally for measuring particle sizes of imaged spheres. A histogram for 1000 counted particles can normally be produced within an hour of starting counting.

Both methods involve the setting of a positive image into alignment with a negative one of the same object. One uses a telescope and the other schlieren photographs.

A simple apparatus is described which can semi-automatically measure thermoelectric voltages of semiconductors from 77 to 600°K while maintaining a sample temperature differential of 10 degC.

The specular reflectance attachment of the Perkin-Elmer 21 range spectrophotometers has been modified to allow measurement of the bidirectional reflectance of fine powders. Bidirectional reflectance spectra of various grain sizes of calcite for the 650-1650 cm⁻¹ frequency range are shown to demonstrate the usefulness of this apparatus modification.

A shutter attachment for photographically aligning a Weissenberg camera is described, together with the procedure by which it is used. Diagrams are given showing that the camera can be aligned horizontally within 0·1 mm of the most favourable position by using it while vertical off-settings of less than 1/16 mm can be detected. Finally, its use for testing the reproductibility of an aligned camera is described.

A process for making lenticular stereograms directly from a hologram is described. The product gives a stereoscopic effect in non-coherent light.

A description is given of a sample holder for rotating a crystal with two degrees of freedom inside a superconducting solenoid at liquid helium temperature. The rotating mechanism has no gear system and allows rotation of up to 20° along any direction.

A standard Pye two-component travelling microscope has been fitted with potentiometers of high linear accuracy which convert the lead screw positions into d.c. voltage signals. Measurements are recorded on punched tape by means of a digital voltmeter-serialiser-tape punch system. Over a measuring area 8 cm square the instrument has an accuracy to ±0·005 mm. The output is semi-automatic and a manual punching facility enables data tapes to be edited in preparation for direct processing by computer. The total cost of components and materials for the unit is less than £2200, which includes £1600 for the data logging system.

Details are given of a cheap, simple and reliable air cooling system which enables a Q-switched neodymium-in-calcium tungstate laser to operate at one pulse per second for long periods.

A GaAs p-n junction diode is forward-biased by a voltage step and the infra-red light emitted during the forward transient is analysed as a function of time at room temperature and liquid nitrogen temperature. It is shown that the radiative response of these devices is more rapid at 77°K than at 300°K and it is concluded that GaAs diodes may be used to obtain fast infra-red light pulses.

A reproducible, easily portable source has been developed to run from the 12 V accumulator of a motor vehicle. It is used to calibrate radiometric instruments in the field. The lowest luminance level is below that of the night sky.

A new method of determining the peak-to-peak displacement of a sinusoidal vibration in terms of the wavelength of light makes use of an interferometer of the Fizeau multiple-beam type, in which the silvered adjacent surfaces of the optical flats act as the two electrodes of a capacitor in addition to performing their function in the interferometer. The two electrodes are surrounded by a third or guard electrode so that the capacitance change between the silvered films which occurs when one of the optical flats is moved can be measured by comparison with a standard three-terminal capacitor in a transformer bridge.

A sinusoidal motion is imparted to one surface of the interferometer and the capacitance change is recorded on a direct-writing oscillograph. This capacitance change is then calibrated by slowly moving the surface of the interferometer in a rectilinear manner and recording by a photoelectric detector the passage of interference fringes past a slit aperture in the optical field. The chart is thus calibrated in terms of interference fringes and the peak-to-peak displacement of the motion readily determined.

A pump suitable for the circulation of aqueous electrolytes is described. To avoid contamination and corrosion, it is totally enclosed and has no metal parts in contact with the electrolyte. The heat input is negligible and it has given trouble-free operation for over 3000 hours.

A method of measuring beam profiles is described. The curves of measured beam intensity of given profiles with a large detector hole are first calculated by representing the detector as a cylinder and the ion beam as a cylinder, a cone, or a truncated cone and then calculating the common volume as a function of their interaxial distance. The original beam profile is given by comparing the experimental curve with the calculated curves.

A beam of light is reflected on to a phototransistor from the surface of a rotating specimen. No additional reflecting or absorbing surfaces are attached to the sample but use is made of the periodic nature of the reflected light intensity due to small surface variations. The rate of rotation of the sample can be determined by detecting coincidence between the modulated phototransistor signal and a similar signal which has been subjected to a known delay by a shift register.

A new method is described whereby extremely short effective gap lengths of ring type reproducing heads used in high resolution magnetic recording instruments can be determined. The method is based on comparison of the frequency or wavelength response of the short gap head with that of a second head of longer gap length which can be conveniently measured by the first null-point method reported previously. Experimental results for the measurement of a head gap of the order of 20 × 10⁻⁶ in are given as an illustration.

The thermoelectric potentials produced by a W-Ta thermocouple were measured at several international secondary fixed-temperature points from 0 to 420°C. The thermocouple may be used in direct contact with boiling mercury for extended periods owing to the very low solubility of these metals in mercury.

A U-tube manometer is described which can be used as a mercury manometer reading up to 120 torr pressure difference or as an oil manometer reading up to 10 torr.

Disks suitable for thinning for electron microscopy may be cut from sheet gold using a stationary jet of 10% hydrochloric acid.

A simple electronic circuit consisting of two transistors, a relay and a power supply, is controlled by a contact thermometer. A two-phase motor drives a variac continuously but very slowly. More or less resistance is switched synchronously with the turn direction of the motor, so that the `on' and `off' periods are always equal, and independent of the energy input.

A description is given of a transistorized shunt regulator which is a plug-in substitute for a high quality gas-filled tube.

An electromagnetic transducer is described in which the moving system is suspended by a thin film of air. This method of suspension results in a high radial and a very low axial stiffness, and enables the transducer to accept an input of large amplitude.

Modifications to a standard Instron machine enable it to measure tensile properties at temperatures of up to 3000°C. Radio-frequency induction heating is used, and the specimen can be surrounded by inert gas or by a vacuum. The apparatus has been successfully used for determining the mechanical properties of iridium single crystals up to their melting point.

Evaporated nickel is found to give a substantially ohmic low-resistance contact on ferrites.