Table of contents

The overall objective of the Home Office Police Scientific Development Branch is the improvement in operational efficiency of the British police forces, by the study and planning of new methods and the development of special-purpose equipment. Scientific staff work closely with senior police officers on a wide variety of subjects. For example, systems research projects include studies of command and control, traffic resource allocation, and major incident record handling, while other groups carry out research on fingerprints and on facial recognition. Equipment work encompasses assessment and development of an extremely wide range of hardware, and four examples are described, namely remote observation, night vision, location of hidden objects, and intruder detection.

A survey is made of the various types of transducer that are available commercially for measuring displacements of less than 50 mm; their performances are compared. The various types of mechanical coupling and conversion that generally limit transducers performance are also compared. The different forms of signal processing and output display are also discussed.

A simple, but potentially accurate, integration technique is described for determining the common-emitter direct current gain beta of a bipolar junction transistor at subnanoampere collector current levels.

A high-temperature furnace capable of attaining temperatures up to 2000K is described. The inner volume of the furnace which contains the sample and the radio frequency coil is kept very small (a few cm³), so that little electric heating power is required (100 W at 200K), and is well insulated guaranteeing good temperature homogeneity. With this furnace, NMR signals have been observed up to 1780K.

The influence of eddy currents on the signal spectrum induced in the pick-up coil of a rotating sample magnetometer is analysed.

In the alternating current method for measuring the electron energy distribution function f(E) in plasmas, the influence of the frequency f of the superimposed signals is studied. When f is near the electron plasma frequency f_pe, the second derivative of the probe characteristics, i_p", is seriously affected, particularly near the space potential, and the distortion of i_p" due to a resonance effect gives a false estimation of the average energy epsilon toward a higher value by more than 20%. The relative error in i_p" is estimated to be of the order of f²/f_pe² at a relatively deep bias, so that except near the space potential f(E) may still be measured accurately with a frequency far above the ion plasma frequency f_pi. Increasing the frequency results in better resolution as the scan rate of the probe voltage becomes rapid.

A sensor is described which has a fast response, is simple to construct and has other advantages over conventional thermistor sensors; it consists of a domestic incandescent gas lighter element.

A pulse techniques based on an inherently safe principle is described for measuring the resistance of electrical circuits in flammable atmospheres. A measurement involves the injection into the circuit under test of a pulse whose energy content is less than the minimum igniting energy of the flammable atmosphere is question. It is shown that the amount of charge which passes through a standard resistor is then a measure of test circuit resistance.

Describes the use of single-crystal sapphire containers used in neutron-scattering experiments on liquid aluminium, copper and nickel up to temperatures of about 1900K. After proper orientation, the containers do not show any prominent coherent peaks, thus reducing the problems with corrections for container scattering considerably.

The frequency of a super-regenerative oscillator can be measured and continuously monitored with a frequency counter suitably interfaced. A simple electrical circuit for the interface is proposed.

A general method for the restoration of true input signal distributions from measured time spectra is described. A factor related to the fraction of start pulses lost in the system busy time periods is introduced.

A torsion pendulum which contains the Collette pendulum as a special one is introduced. Uncoupling of damped, coupled oscillators associated with the three-component pendulum yields a simple type of symmetric arrangement different from the Collette pendulum. This new pendulum is shown to be equivalent to two pendulums of the Ke and inverted types.

An open Stark cavity is described which allows for the detection of Delta M=+or-1 microwave transitions. High and very homogeneous electric fields can be applied using a simple construction technique. A microwave absorption detection limit of less than 10^-6 m^-1 is obtained. These features in combination with a long infrared transmission path render the cavity especially suitable for Stark-shifted infrared-microwave double-resonance experiments. Under certain conditions Lamb dips in the infrared absorption can be observed.

The velocimeter described is intended for measuring given lengths of iron wire in a rolling mill. The device is based on the conventional local oscillator heterodyne mode. To prevent the disappearance of the Doppler signal caused by movements of the wire in a direction perpendicular to the wire and laser beam, a broadening of the beam by means of a cylindrical lens is also proposed. The maximum Doppler frequency of the system is about 50 MHz, and the corresponding surface velocity is 66 m s^-1 at a measurement angle of 14 degrees .

A method is described for automatically processing data derived from photographs of trace particles in liquids contained in laboratory systems to obtain a two-dimensional velocity field. The method includes the interactive use of a numerical velocity analysis scheme enabling a high density of observations to be used without ambiguity. The experimental and numerical techniques are described with particular reference to the determination of the flow field in a rotating annular convection chamber. Assessments of the accuracy of the technique are made using simulated data and by comparison with manually extracted data.

A study of the two-spot skin friction meter covering effects such as misalignment, skin friction variation and three-dimensionality of flow, is described. Conditions in which the oil flows under both gravity and skin friction forces are covered. This has practical applications, particularly in measurement of very low skin friction values. It also permits a study of the effects of pressure gradients which confirms that the method is applicable in high adverse gradients.

A technique for measuring pressure variations whose magnitudes are typical of a weak shock is described. The technique has high spatial resolution (about 10^-4 m), and can have high temporal resolution (of the order of 10^-9 s).

A very simple modulator is described which enables accurate flow birefringence and extinction angle measurements to be made at low rates of shear. The modulator consists of a glass bar with U-shaped cross-section, which is excited in its lowest eigenfrequency of transverse vibration. In this way one obtains a periodic birefringence with easily adjustable amplitude and a suitable frequency for electronic processing.

It is shown that the high-resolution electrostatic Mollenstedt energy analyser is capable of measuring high-voltage fluctuations. The sensitivity is shown to be better than 10^-6 at an accelerating voltage of 25 kV. The analyser can well be applied to ripple measurements. Since fluctuations in accelerating voltage affect the energy of the electrons, the high-voltage ripple may be evaluated from measurements of the energy distribution.

The feasibility of constructing a third-order correlator around a normal second-order digital correlator is discussed. The principle is valid for the measurement of the nth-order moment of stationary random signals. Applications dealing with both deterministic and random signals are described.

By using a heating law of the form 1/T varies as t, the analysis of thermoluminescence glow curves is made considerably easier. Two simple circuits for achieving this are described that may be switched into the ramp generator circuit of a time-linear temperature controller.

An automatic NMR magnetometer covering the magnetic field range 0.1-3.2 T is described. Instead of continuous tracking of the magnetic field by a conventional feedback loop technique, a new approach to triangular frequency modulation of the marginal oscillator is used in this instrument, ensuring symmetrical frequency deviation about the resonance position. Direct digital display of the magnetic field value is obtained by digital counting of the swept frequency with a suitable counting gate time. The particular advantage of this system is that the restrictions characteristic of automatic frequency control feedback loop systems, as well as for adiabatic passage conditions, can be omitted. The magnetometer is a data-sampling system with a sampling frequency of 43 Hz and a typical tracking rate of about 0.12 T s^-1. The maximum search time is 0.5 s. Because of the good dynamic properties and measurement accuracy approaching 10^-6 T and lack of modulation coils in the probe, the instrument is particularly suited for the measurement and calibration of magnetic fields in ESR and NMR spectrometers.

An apparatus for magneto-optical measurements is described which has the following features: The sample holder, machined in aluminium and supplied with a copper sheath, balances the thermal expansion of the materials and restricts the sample shift to 40 mu m when the temperature is varied between 1.7 and 300K; the chamber containing the superconducting coil has a small capacity (5 l of liquid helium); two samples can be fixed to the sample holder and can then be studied by cooling the cryostat only once. Some experimental results are given as an illustration.

The authors present the operational principles and experimental results obtained when using a single crystal of beryllium immersed in liquid helium as an indicator for measurements of strength and inhomogeneity of high magnetic fields. The indicator is tested in fields up to 1.19*10⁷ A m^-1 (15 T) generated by both water-cooled and superconducting electromagnets. It is shown that this indicator is useful for fields higher than 2.15*10⁶ A m^-1 (2.7 T) and for gradients up to 1.19*10⁸ A m^-2 (150 T m^-1).

Describes the optimisation of superconducting solenoids with constant current density and with radially subdivided sections having graduated current density using sixth-order compensation. These coils are compared with other types of compensated coil in the effectiveness of the superconducting conductor volume and the higher-order error coefficients.

Techniques are described for the measurement of magnetisation and susceptibilities to accuracies of +or-5*10² A m^-1 and +or-3*10^-5 respectively in 17 T, 0.7 ms pulsed magnetic fields. The unique features are the use of copper-beryllium wire in the search coils to reduce eddy current effects and a sinusoidal field pulse to obtain symmetric magnetisation curves from which an origin can be found, independent of remanent magnetisation and long-term drift.

Describes a simple experimental geometry for studying polarisation effects in angle-integrated photoemission with synchrotron radiation. To illustrate the feasibility and accuracy of the approach, experiments which reveal wave function symmetries of valence band states in GaSe are discussed.