Table of contents

It is shown experimentally that the beat frequency of a molecular beam Zeeman maser may be phase-locked to a periodic modulation of either the molecular or cavity Q.

Calculations of the pressure in the liquid core of a freezing water droplet and stresses in the surrounding ice shell are made. The theory is developed in terms of two mechanical models for the ice. The first model assumes it is elastic and the second that it is linearly viscoelastic. The calculated pressure development is compared with that measured in drops one centimetre in diameter. It is concluded that the viscoelastic theory is sufficient for describing the pressure growth, but overestimates the magnitudes of the stresses in the shell. It is suggested that a treatment using a non-linear dependence of strain rate on stress would yield a more realistic stress distribution.

A series of experiments has been performed to measure the conversion coefficient for longitudinal to Rayleigh wave scattering when the former is incident on a rough section of an otherwise plane free surface. The waves are harmonic in time, and the roughness takes the form of linear symmetric grooves. The results are used to test the predictions of perturbation theory, and to find the conditions under which they may be regarded as satisfactory. Apparently the theory breaks down when the surface slope exceeds 25° if the linear extent of the irregularity is small.

Outside the range of application of the theory (ie when the extent of the irregularity becomes large) the experimental results are interpreted in terms of energy balance, and approximate conversion coefficients are derived. The conclusion is that Rayleigh waves are attenuated to about a tenth of their energy in travelling a distance of ten wavelengths across regular grooves a wavelength in width and with slopes of 10-25°.

The magnetic pressure in a cylindrical wire has been calculated for constant current, steady-state sinusoidal current, and the pulse current of an overdamped discharge. As previously shown by Haines (1959) for conductors carrying time-varying currents, an inverse current density distribution is produced corresponding to an `inverse skin effect'. Maninger (1959) and Haines predicted that the inverse skin effect also creates an inverse pinch effect, thereby producing an outer shell of magnetic tension in the wire. A quantitative analysis for a pulse current-carrying conductor shows that the inverse pinch effect becomes dominant over the normal pinch effect in later time instants of the current pulsing. The distribution of the magnetic pressure in a rectangular rod carrying a constant current has been calculated for different ratios of thickness and width. The validity of this solution for thin foils is discussed for the case of applied alternating currents.

An evaluation is made of an iterative method for determining the amplitude and phase from the image intensity recorded in optical systems. The method, which requires two images recorded at different lens defocus values, is tested with simulated data subject to error arising from the photographic recording of the image. In the case of error-free data, the solution for the phase distribution appears to be indeterminate to within a constant. The results for photographic noise levels of up to 20% of the maximum image intensity reflect the small effect of error on the calculated phase distribution. The calculation of phase distributions for both symmetric and asymmetric amplitude-phase distributions shows that the use of two images, taken at defocus values differing by about 100 nm in electron optics and about 1 mm in optics (depending on the numerical aperture of the objective lens), may be used to determine the complex object wave-function in both dark-field and bright-field optics.

In the evaluation of an iterative scheme that determines the amplitude-phase distribution of an image from two image intensity distributions recorded at two different lens defocus values, the effects of various sources of error on the solution for the phase distribution are examined, namely, a background signal superimposed on the image intensity, a mismatching of the two images and an error in determining the defocus difference between the two images. In electron optics, a background intensity, arising from (say) inelastic electron scattering, corresponding to 10% of the maximum image intensity has a significant effect on the calculated phase distribution. In high-resolution electron microscopy, achieving a potential image resolution of 0·1-0·3 nm, a mismatching of the two images by 0·1 nm is acceptable, corresponding to a misalignment of the two electron micrographs by 50 μm at an electron-optical magnification of 500 000; and a defocus error of 10 nm, in a total defocus difference of 100 nm between the two images, does not severely distort the solution for the phase distribution. The combined effect of photographic noise (10-20% of the maximum intensity), mismatching (0·1 nm), defocus error (10 nm) and a background (6% of the maximum intensity) on the calculated phase distribution gives an indication of the maximum error that can be tolerated in an experimental test of the method. The magnitude of the maximum error that can be tolerated may be scaled to the resolution required in the phase solution and the wavelength of the radiation used; in optics with a potential image resolution of 10 μm, a mismatching of images by 5-10 μm (at unit magnification or 0·5-1·0 mm at an optical magnification of 100) is acceptable and a corresponding defocus error of 100 μm for a defocus difference of 1 mm (depending on the numerical aperture of the objective lens).

A technique for simulating charged particle tracks in spark chambers is described. The method is extremely versatile, and several of its applications are outlined. The technique depends on the production of long-range ultraviolet photons in a spark. The mean free path of these photons in a neon-helium gas mixture is measured as 9·0 cm.

A time-of-flight mass spectrometer has been used to examine the dissociation of nitrous oxide in a glow discharge. The dissociation arises from the direct electron impact reactions e+N₂O->N₂+O+e and e+N₂O->N+NO+e. The overall dissociation coefficient was found to be 7·3 × 10⁻¹⁷ cm² for 2·2 Torr N₂O (E/p similar 30 V cm⁻¹ Torr⁻¹) and to increase by nearly an order of magnitude when the N₂O is mixed with argon. The discharge product NO significantly alters the discharge conditions and is itself considerably dissociated.

A sampling technique with pulsed probe bias is described which enables time-resolved electron energy distribution measurements to be made in higher-current discharges (similar 300 mA cm⁻²) in which the probe would normally become incandescent. The method enables the probe temperature to be significantly reduced, eliminating errors due to large variations of the probe surface workfunction and thermal emission. The method is illustrated with measurements made in a neon discharge operating above the Pupp limit.

Less than half of the heat generated in a TIG welding arc (typically 1600 W at 16 V, 100 A) is transferred to the workpiece (anode). Convection, conduction and radiation from the gas occur over the whole of the arc region, but they represent relatively minor contributions to the total heat balance. The principal anode heating and cooling mechanisms involve electron and space-charge effects at the surface. These electron effects are evident in the workfunction (typically 4·5 V) which is dominant, the electron thermal energy transfer (1 V) and the anode fall (2 V), and they are concentrated in the restricted anode current spot so they may be considered as a localized heat source.

Evaporation is most intense from the anode spot region and carries away some heat. However, most of the vapour condenses on the cooler, outer regions of the anode surface covered by the arc. In this way, heat is redistributed and diffused over a wider area. Vaporization effects explain the differences between previous measurements with cooled copper anodes (in which 80% of the arc power is transferred to the metal) and ones with practical, molten-steel welds (less than 50% heat transfer).

It is argued that experimental results for the flow of certain elasticoviscous liquids through slits and capillaries are consistent with those obtained by other means in as much as they indicate a second normal stress difference which is negative and smaller than the first normal stress difference. Existing interpretations of these experimental results, which predict a large positive second normal stress difference, are in error because they assume that the centreline pressure at the exit of the capillary is zero.

Some assumptions in the author's earlier (1973) work on the gas atomization of liquids have been relaxed; also the work has been extended to cover the case of supersonic gas speeds. A prediction of the drop size of a spray can be made and good correlation with experimental work is found. Finally, for molten metals, a graphical method of prediction is developed.

Laser light scattering and optical mixing spectroscopic techniques have been used to study the dispersion relation of kHz frequency thermally excited capillary waves on the free surfaces of p-azoxyanisole and p-azoxyphenetole. The experimentally determined dispersion relation agrees well with the predictions of hydrodynamic theory, both with respect to the frequency and to the damping of the waves.