Table of contents

Oscillographic measurements have been made of the variation with time of the voltage drops in long spark channels conducting aperiodic impulse currents rising to a peak in about 1/4 μsec. and decaying to half-value in from 10 to 85 μsec., with peak currents ranging from 60 to 700 amp. The lengths of the spark channels have been measured photographically, and the mean voltage gradients along the channels have been estimated. Results are given for sparks in air, nitrogen, oxygen and hydrogen at atmospheric and reduced pressures. Some preliminary measurements are described for sparks in water and oil, and for sparks in glass tubes. The voltage gradients in the various gases, with the exception of hydrogen, are independent of the peak current, but are influenced by the rate of current decay. The results are correlated with the current densities in spark channels, as recorded in a separate investigation.

A rotating mirror camera with a temporal resolution on the film of 1.5 mm/μsec. has been used to determine the rate of expansion of long gaseous spark channels during their first 10 μsec. of growth. The spark channels conducted aperiodic impulse currents in the range of 60 to 500 amp. peak, which attained their peak values in about 1/4 μsec. and decayed to half-value in 10 or 28 μsec. Sparks have been investigated in air at atmospheric and reduced pressures, and in nitrogen, oxygen and hydrogen at atmospheric pressure. The measured areas of cross section of these spark channels bear a linear relation to the peak currents, except in hydrogen, and are little influenced by the rate of current decay in the range studied. Measurements have also been made with the rotating mirror scanning an image of the spark across a photomultiplier connected to a high speed oscillograph; by this method a direct record is obtained of the radial light distribution across the spark channels. The mechanism of spark channel expansion is discussed, and estimates are made of the average ion densities in spark channels.

In order to determine whether any discontinuous change in electrical properties takes place when a liquid is supercooled, measurements have been made on the resistance of liquid gallium. No such discontinuity has been found, the resistance varying linearly with temperature over the range investigated.

The results yield values for the resistivity and temperature coefficient of resistance of liquid gallium.

Permanent gas counters are discussed and a new quenching circuit is described Quenching times of about 45 μsec. have been used and plateaux with slopes better than 0 05% per volt obtained A formula for the correction of counting rates for losses caused by the counter dead-time has been confirmed within wide limits. It has been found possible to measure counting rates as high as 5 × 10⁵ counts per minute with an accuracy closer than 1%.

The problem of cavitation produced by ultrasonic vibration is examined theoretically. Equations are developed which describe the motion of a gas-filled cavitation bubble in a liquid medium subjected to alternating pressure; the case of an empty cavity is also considered Information is obtained concerning the distribution of fluid pressures and velocities in the medium near the bubble surface during the motion It is suggested that these theoretical conclusions may be used to show how the intensity of the various effects of cavitation will depend on ultrasonic frequency and intensity. In particular, it is predicted that all cavitation phenomena will diminish and finally disappear as the frequency is raised The important part played by the nucler from which the cavitation bubbles grow is emphasized.

The definition of colour temperature is explained and the approximations of the spectral distributions of energy of lamps to those of black-body radiators are indicated. The value of colour-temperature measurements to the colorimetrist is outlined and difficulties in making the measurements are dealt with. The use of photoelectric devices for colour-temperature measurements are mentioned

The equipotential lines in a bipotential electron lens exhibit a fine structure which is revealed by application of the relaxation method to the solution of the Laplace equation in cylindrical coordinates The asymmetry of the potential field between the two edges of the cylinders is confirmed by the same method and is established to be of the order of 2% in one particular case

The dependence of the resolving power of the asymmetrical bell-shaped magnetic field on the lens parameters is examined for varying degrees of field asymmetry It is shown that a high asymmetry index favours high resolution, and a preferable value of lens power is specified.

By suitably reinforcing the lens field with that of a small air-cored coil, the theoretical resolving power of the lens is increased. On account of the small size of the coil, the coil current must be pulsed intermittently if an appreciable reinforcing field is to be produced.

The basic factors governing the design and operation of a suitable coil are discussed

Aberration formulae given previously, together with new ones for astigmatism and secondary spherical aberration, are put into forms which are suitable for computation and simplify the treatment of field lenses. A method of comparing the formulae with ray tracing is given, and the results for a triplet objective are discussed.