A study of the formation of vortex rings when a liquid drop falls into a stationary bath of the same liquid has been made. The investigation covered liquids with a wide range in surface tensions, densities and viscosities. The results confirm the reported existence of optimum dropping height from which the drop develops into a superior vortex ring. The optimum heights are analysed, by a photographic study, in terms of the liquid drop oscillation. It is found that vortex rings are formed best if the drop is spherical and changing from an oblate to a prolate spheroid at the moment of contact with the bath. A Reynolds number has been determined for vortex rings produced at optimum dropping heights; these numbers are approximately 1000. A possible mechanism for the ring formation is suggested.

Parameters that promote rapid formation of aligned high-T, phase in the high-temperature superconductor (Bi, Pb)2Sr2Ca2Cu30x (Bi-2223) powder in tube superconducting tapes have been studied. A rapid phase formation was achieved by two means. Firstly, frequent mechanical pressings during the early stages of the heat treatment were applied. The pressings counteract the expansion that occurs during sintering, increase grain connectivity and reduce diffusion distances. Secondly, the fraction of process time within the thermodynamically stable region of the Bi-2223 phase was maximized by ramping of the oxygen partial pressure during heating and cooling. In addition, the starting phase composition of the superconductor powder was found to have a critical influence on the reaction speed. A complex phase composition gave the fastest formation rate of 81-2223 phase. Within 30 h of processing time, critical current densities exceeding 12 kA c w 2 (1 WV cm-', 77 K, 0 T) were obtained compared to 2 4 kA cm-> without the above measures. Also, the effect of variations in oxygen paltial pressure, pressing schedules, paltial melting and a two-component starting powder based on (Bi, Pb)2Sr2CaCu20, were investigated for this process.

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A variable temperature apparatus is described which uses a thermal conductivity measurement technique for the determination of superconducting ac power loss. The technique consists of measuring the increase in temperature of a thermally isolated sample carrying an alternating current. Accurate measurements over a relatively large temperature range are possible. Loss data on a 13 cm long, single filament NbTi superconductor are presented at various temperatures and currents.