It is shown that a bubble of gas or liquid, immersed in a liquid medium and subjected to an electric field between parallel plate electrodes, assumes the shape of a prolate spheroid in the direction of the field. Expressions for interfacial traction between two fluid dielectrics, if derived by taking into account electrostriction (Stratton 1941; Smythe 1950), are shown to be in disagreement with experimental results and must therefore be considered incorrect. Using expressions for interfacial traction not involving electrostrictive terms, equations are derived for the dependence on electric stress of the elongation of compressible (gaseous) and of incompressible (liquid) bubbles immersed in an insulating liquid. These show that as the field strength is increased, conducting bubbles, and also non-conducting bubbles for which the permittivity of the bubble exceeds twenty times the permittivity of the medium, elongate until a critical shape is reached when the bubble becomes unstable. For conducting bubbles the critical shape corresponds to a ratio of the major to the minor semi-axis of 1*85. Bubbles of permittivity ratio lower than 20 have no critical shape, the axial ratio increasing indefinitely with increase of field strength. There is satisfactory agreement between theory and experiment. The implications of these results with regard to electrical breakdown of liquids are discussed.
A study was made of electrical breakdown of a liquid (hexachlorodiphenyl) the viscosity of which is very dependent on temperature. It is shown that in uniform fields breakdown results from formation and growth of a vapour bubble in the liquid. This was established by direct microscopic observations of the ‘breakdown event’ at room temperature and by measurements of times to breakdown as the viscosity was changed by five orders of magnitude between room temperature and 56.5 °C. When the time of voltage application is too short for the vapour bubble to grow to its critical size then the breakdown strength is higher than that obtained under direct voltages. Under 10/50 μs impulses the breakdown strength of hexachlorodiphenyl at room temperature was 5 MV/cm. It is suggested that vaporization was initiated by development of points of zero pressure in the liquid. Assuming that, in an electric field, development of points of zero pressure resulted from the presence of submicroscopic particle impurities in the liquid, a simple expression was derived for the onset of vaporization. It is shown that this expression gives good predictions for the known dependences of breakdown strength of n -hexane on both temperature and pressure and for the variation of breakdown strength of aliphatic hydrocarbons with molecular weight. It is further shown that the time required for a vapour bubble in n -hexane to grow to the size at which breakdown occurs is comparable with the experimentally measured formative time lag.
In void-free oil-impregnated paper, gas evolution starts at a critical stress which is markedly dependent on the degree of dryness of the paper. The gas first formed arises from decomposition of water in the cellulose, the nature of the impregnant having little effect. Subsequent more rapid gassing resulting from decomposition of the oil is a secondary process depending on ionization within gas bubbles previously formed. Study of the fundamental primary process suggests that water absorbed by the cellulose is ionized by electron bombardment in regions of high stress and is then decomposed electrochemically.
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