Abstract
LONDON. Physical Society, November 8.—Mr. T. H. Blakesley, vice-president, in the chair.—A paper on a voltameter for small currents was read by Dr. R. A. Lehfeldt. The instrument consists of a capillary tube about 25 cms. long completely filled with mercury with the exception of a bubble of mercurous nitrate solution about 1 cm. long placed near the middle of the tube. Connection with the two mercury columns is made by means of platinum wires passing, through the side of the tube. To use the instrument it is placed in a vertical position, the anode being at the top, and the quantity of electricity which passes through is measured by the change in volume of either electrode. In a test experiment the change in volume was measured by means of a micrometer, and agreed within O·6 per cent, with the amount deduced from the known value of the current. It is necessary that the currents should be small, so as to avoid complications due to polarisation. The chairman pointed out that the presence of air in the tube would: render the readings inaccurate, and asked if it was necessary to apply any temperature correction. Dr. Lehfeldt said that it was quite easy to seal the tube without admitting air, and the temperature correction was negligible.—A note on a paper by Prof. Fleming and Mr. Ashton entitled “On a Model which Imitates the Behaviour of Dielectrics,” by Dr. J. Buchanan, was read by the secretary. The action of this model depends on the viscosity of a liquid, and the diagrams derived from it show by their form that the motion of the pencil which traced them approximated closely to what may be expressed by the term “motion of a viscous fluid by diffusion.” In other words, the displacement curves obtained from the model and their derived velocity curves are of the same form as the graphs of certain solutions of Fourier's well-known equation dv/dt = K d2v/dx2. Lord Kelvin has shown that the potential and the current at any point in the wire of a cable can be expressed by appropriate solutions of this equation, and in the same manner by the use of solutions of this equation the diffusion of electricity into or out of the dielectric of a condenser can be treated. It appears, therefore, that the motion of the model and the diffusion of electricity in a dielectric are subject to one and the same mathematical law. The author suggests that the inventors should obtain hysteresis diagrams by cyclical loading of the springs. Prof, J. A. Fleming said he was glad that Dr. Buchanan had drawn attention again to the model because there were points about it which might be amplified with advantage. After giving a short description of the apparatus he said that Dr. Buchanan had shown that mathematically the theory of the model was the same as that of diffusion in a cable, and he suggested that there might be something more than mathematical analogy. Prof. Fleming referred to the discussion on the original paper in which Prof. Ayrton asked in what respect the model served its purpose better than a twisted wire. A twisted wire cannot represent the properties of a dielectric, because if twisted beyond the elastic limits there is a permanent set. There is no permanent set in the present model. He would like to know if a dielectric has a true conductivity, and suggested that experiments should be made by subjecting a dielectric to constant electric pressure at constant temperature, for years if necessary, and observing whether the curve of current becomes asymptotic to the zero line or to a line parallel to it. The model could be made to represent a conduction as well as a displacement current by so arranging the bottom piston that it could descend but not return. The fact that the movements of the model were similar to the diffusion of current in a cable suggested that the process of conduction in a metal was similar to that of displacement in a dielectric—Mr. J. Macfarlane Gray read a note on the numerical value of the “characteristic” of water. The author referred to a paper on thermodynamics which he wrote twenty years ago and in which he supported the theory of a granular ether under enormous pressure. This theory easily explains the properties of bodies. There is a numerical characteristic for every substance in the state of vapour. This characteristic can be deduced from an analytical expression involving certain physical data which must be experimentally determined. His original number for water was 25·30693, but later experiments by Lord Rayleigh on the weight of hydrogen have altered this number to 25·33776. The author's original value for the absolute specific heat of water was 124960 “mms. lift at Paris,” but recent experiments of Callendargive 126230. According to the author's theory, water commences to freeze at 95° F. and the variation of the specific heat of water at low temperatures is due to the latent heat of ice. The formation of ice particles also explains the peculiar changes in volume of water as it cools to the freezing point. The chairman asked if this theory could explain the fact that water can remain liquid below 32° F. Mr. Macfarlane Gray said it could.
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Societies and Academies . Nature 65, 46–48 (1901). https://doi.org/10.1038/065046b0
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DOI: https://doi.org/10.1038/065046b0