Societies and Academies

Abstract

LONDON Royal Society, March 3.—J. C. McLennan, A. C. Burton, A. Pitt, and J. O. Wilhelm: The phenomena of superconductivity with alternating currents of high-frequency. With currents of frequency 1.1 × 107 per second a coil of lead wire showed an abrupt loss of resistance, of relatively large amount, at a temperature that appeared to be slightly lower than the critical temperature 7.2° K. characteristic of the transition to superconductivity, found for the same wire with direct ctirrent. With a coil of tin wire, drawn to a diameter of 0.3 mm., it was found that with direct currents the resistance of the coil began to decrease abruptly at 3.76° K. and disappeared completely at 3.70° K. Experiments with the same coil with currents of frequency 1.lxl07 per second gave for the corresponding temperatures 3.67° K. and 3.61° K. Further experiments with higher frequencies revealed depressions of the critical transition temperature increasing in amount with the frequency. Extrapolation of the transition temperature-frequency curve, which appeared to be linear for the higher frequencies, gave 109 per second for the frequency corresponding to 0° K. With tantalum wires results were obtained similar in character to those found with wires of tin and of lead. Polarisation and orientation phenomena are involved in the production of the superconducting state in metals. This electrical state appears, in part at least, to be somewhat analogous to the saturated magnetic state obtainable with ferromagnetic metals. (See also NATUBE, 128, p. 1004; 1931.)—G. I. Taylor: The transport of vorticity and heat through fluids in turbulent motion. The theory that the dynamics of turbulent motion should be regarded as an effect of diffusion of vorticity rather than as a diffusion of momentum was put forward by the author in 1916, and the particular case when the whole motion is limited to two dimensions was then discussed. The analysis is now extended to three-dimensional motion, and it is shown that the ‘momentum transport’ theory of Reynolds and Prandtl agrees with the ‘vorticity transport’ theory in one case only, namely, when the turbulent motion is of a two-dimensional type, being confined to the plane perpendicular to the mean motion. When the turbulent motion as well as the mean motion is confined to two dimensions, the vorticity transport theory yields results which are quite different from those predicted by the momentum transport theory. The distribution of temperature and velocity in the wake behind a heated obstacle for the case of two-dimensional motion, when the turbulent motion is confined to the plane of the mean motion, is in accord with the vorticity transport theory.—A. Page and H. C. H. Townend: An examination of turbulent flow with an ultramicroscope. Minute particles present in tap water were intensely illuminated and viewed against a dark background. These particles were small enough to show the Brownian movement when the fluid was at rest. The illuminated particles were used to follow the motion of the fluid, and enabled the maximum valuen u1, v1 and w1 of the three components u, v, and w of the velocity disturbance at any point and the distribution of the mean velocity U across the pipe to be made without introducing any instruments into the fluid. At the centre of the pipe, u1 v1 and w1 were approximately equal. As the wall was approached,v1/U obtained from the velocity disturbance normal to the wall decreased to zero, whilst u1/ U and u1 U increased. At the wall itself, whilst the flow tended to the laminar type, the motions of particles in the laminæ were sinuous, even to within a distance of 1/40,000 in. from the wall. No particle was seen to move in a rectilinear path.