Table of contents

An experimental method of determining the peak r.f. voltage across the circuit interaction gap of velocity modulated electron beam devices is described. The method uses a parallel plate type of retarding field analyser to measure the energy of the fastest electrons emerging from the exit plane of the interaction gap.

The results can be used to determine the shunt conductance and R/Q value for the resonant circuits. The method is capable of considerably greater accuracies than conventional techniques. It is of particular value when gap voltages are measured under pulse conditions, and when the cavity is subject to unpredictable loading effects arising from the beam or from multipacting.

A low energy ion source for bombarding electron microscope specimens is described. The source is capable of delivering a current of 0.3 μA of 100 eV argon ions to an area 2.0 mm in diameter.

Absolute measurements of thermal radiation have been obtained from the transient temperature rise of a copper block. The construction of the instrument, the manner of its use and the method of calculating the incident intensity are described. Values obtained in this way are in satisfactory agreement with those obtained using a water flow calorimeter.

An optical method is described of obtaining a silhouette image of a specimen during a tensile test carried out under high hydrostatic pressures of up to 12 200 atm (80 ton in^-2). The results of tests on various windows are also given. The optical method can be used for observing in silhouette other phenomena under high hydrostatic pressure.

The National Physical Laboratory plasma-metal junction harmonic generator is based on the non-linear properties of the cathode spot of an arc established in a gas at high pressure. To inhibit transfer of metal from one electrode to the other the arc must be maintained by a direct current source which reverses polarity with great rapidity several thousand times a second and it is to this end that the high speed power switch has been designed. The device is capable of delivering a square wave of greater than 100 v peak at up to 0.5 A with a rise time of 0.2 μsec.

Two new instruments are described for investigating the rheological properties of clay-water mixes. One involves the axial compression at constant rate of a cylindrical specimen between oiled parallel plates; the other subjects an extruded cylindrical rod to sinusoidal strains in torsion at frequencies in the range 0.01-0.16 c/s. In each case the response of the specimen is measured by the amount of deformation of an elastic element using a Philips displacement pick-up, and the results are recorded automatically on an X-Y recorder as load-compression or torque-deflection curves respectively.

A nuclear resonance pulse apparatus is described which is suitable for broad absorption lines as found in solids as compared with previously described equipment only suitable for liquids. The equipment gives a better than 1 μsec 2.5 kv, 20 Mc/s pulse and reads a 100 μv nuclear signal 4 μsec from the beginning of the pulse. The design of the modulator, oscillator, receiver and probe are discussed in detail. The principal difficulty is in achieving satisfactory pulse cut-off. The problems arising are of general interest.

The application of suitable feedback techniques to a transistor amplifier enables input resistance values of 250-500 MΩ to be readily obtainable, with extension up to 2000 MΩ at the expense of noise performance. Typical performance figures are 250 MΩ over a frequency range 1 c/s-40 kc/s, with unity voltage gain and a noise level of about 0.5 mv peak to peak. Extension of the frequency range to direct current may increase the noise to several hundred millivolts.

Temperature-time characteristics of tungsten filaments heated electrically under constant voltage in vacuum have been analysed. The analysis is carried out over the temperature range 300-2500° K, taking into account the actual variations with temperature of the various parameters involved, as reported by Jones and Langmuir (1927). The analysis leads to the conclusion that the temperature-time relationship is exponential throughout the range. The time constant is shown to be proportional to the diameter of the filament and T_f^-4.2 where T_f is the final temperature of the filament. The results of the analysis are applied to derive the voltage variations (continuous and discrete types) required to keep the transient current within specified limits during the rapid switching on of filaments as met with in high power thermionic valves.

A vibrating capillary device, consisting of a hypodermic needle vibrated at its resonant frequency by an electromagnetically driven diaphragm, produces controllable and very uniform streams of drops of radius down to 15 μm. The size and frequency with which the droplets are produced depend upon the flow rate of the liquid through the needle, the needle diameter, its resonant frequency and the amplitude of oscillation of the needle tip. The device is being used to study the collision and coalescence of small water drops in air.

It is shown that, in the design of magnetic probes for plasma field measurements, attention must be given to the space resolving power of the coils. A technique is described, which, by mechanical means, gives an exact alignment of the individual coils in a multiple probe without any need for further manual adjustment. Integration of the signals by means of transistorized Miller integrators, placed at the probe end of the transmitting cables, gives a good signal-to-noise ratio. The described system has an overall sensitivity of 1 mv G^-1 at an RC time of 8 msec, and the bandwidth extends to over 2 Mc/s.

The theory of transmission oximetry is extended to take into account the scattering of light by whole blood, and conditions are formulated under which a single-scale oximeter may be constructed. An instrument is described which has been designed particularly for cardiac catheterization procedures. After setting up on an initial sample of blood, a series of readings can be taken in rapid succession without the need to manipulate controls.

The theoretical approach is confirmed by the ability to deduce from the performance curves of the oximeter (operating on whole blood) values for the absorption coefficients of haemolysed blood which agree well with published spectrophotometer values.

An apparatus is described which allows the mass spectrometric analysis of rather involatile materials. Modifications have been introduced which facilitate this type of investigation.

An assembly which will contain targets up to 16 in. long is described. A means of emptying and refilling the target rapidly during an experiment is explained.

An adiabatic calorimeter used in conjunction with a pneumatic sample transfer system is described. Results indicate that useful measurements may be made of nuclear heating rates above 20 mw g^-1 and that by improving the present arrangement the useful range might be extended to 5 mw g^-1.