Abstract
LONDON. Royal Society, January 22.-Sir J. J. Thomson, president, in the chair.--"Prof. E. G. Coker and K. C. Chakko: The stress-strain properties of nitro-cellulose and the law of its optical behaviour. The physical properties of nitro-cellulosc are studied from its behaviour in tension, whereby values of Young's modulus and Poisson's ratio are obtained and the form of the load-extension curve is determined. The optical properties of the transparent material are observed, with special reference to its behaviour under load; and it is shown, by observations with a comparison beam not stressed beyond elastic limit, that the relative retardation produced between the two components of a polarised beam is consistent with a linear optical stress law, which holds up to stresses of about twice those at the elastic limit. These results are confirmed by observations of the retardation, bands produced in a polarised spectrum by a beam under uniform bending moment. The stresses and strains are deduced on the assumption of a linear stress-optical law, and stress-strain curve so obtained is found to agree with purely mechanical measurements of a tension member.-S. Marsh: Alternating-current electrolysis. The behaviour of platinum, gold, and nickel electrodes during- the passage of an alternating current of 25 to 80 cycles per second has been examined. The electrolytes employed were dilute sulphuric acid and barium hydrate solution. Curves representing the relation between volume of gas evolved and time of passage of current are of two distinct types: (a) One type resembling "saturation-current "curves in radio-activity. This effect is shown clearly in the cases of platinum and gold in an acid electrolyte. (fr) A second type in which the rate of evolution of gas falls off with time until ultimately a steady rate of evolution sets in, decreasing in value with increasing frequency of the alternating current. Two possible explanations of the phenomena are discussed: (a) Adsorption of hydrogen at an electrode during one half-period, followed by recombination with oxygen in the succeeding- half-period. (&) Oxidation of the electrode bv the oxygen of one half-period, followed by reduction of the oxide bv the hydrogen of the succeeding half-period. Evidence is given that the oxidation theory successfully explains the effects with gold and nickel. In the case of platinum it is believed that oxidation plays a prominent rdle, though adsorption also may be effective in this case. It is shown that the electrodes have an initial surface activity in promoting recombination, which activity increases (a) with frequency of alternation, and (&) up to a maximum value with the time of passage of the current. If the current density is less than that corresponding to this maximum activitv, then sooner or later evolution of gas ceases. If the current density is greater, then after a time gas is evolved at a steady rate. - W. H. Eccles and J. H. Vincent: The variations of wave-length of the oscillations generated by three-electrode thermionic tubes due to changes in filament current, plate voltage, grid voltage, or coupling. When electrical oscillations are sustained in a circuit comprising inductance and electrical capacity by aid of a three-electrode thermionic vacuum tube of the kind used in wireless telegraphy, the frequency of the oscillations and the wave-length of the radiation depend principally upon the values of the inductance and the electrical capacity, but also partly upon the resistance in the oscillatory circuit; upon the voltages of the various batteries in use; upon the temperature of the filament supplying the electrons; upon other properties of the vacuum tube; and upon the coupling between portions of the circuit associated with the grid and the anode. The object of the present investigation was to study experimentally the effects of altering each of the chief variables, with the view of finding the conditions most favourable for the production of continuous oscillations of constant frequency. For this purpose two circuits were sustained in oscillation at nearly' the same high frequency, namely, about 120,000 vibrations per second, and the pudible beat between these frequencies was observed. Then chang-es made in one circuit alone caused variations of freauency that were measurable by acoustic observation of the beat-note. The preliminary inves-tifations showed that variation of the heating- current of a filament was the most fertile source of erratic changes of frequencv, and resulted in the discovery that increase in the filament current of one tube produced at low values of current a decrease of frequencv, and at higher values an increase of frequency, while at a certain value of filament current the frequency had a stationary value. This phenomenon provides a method of setting an oscillation generator so as to produce a vibration of frequencv constant to less than one part in 100,000. Provided with this knowledge, the other problems enumerated above were attacked. In an apparatus in which the inductance was eight millihenries, the electrical capacity 250 electrostatic units, and the wave-length 27^0 metres, it was found that raising the voltage of the anode battery from 130 to 140 increased the wave-length by 6 metres, and raising the grid voltage by I increased the wavelength about 10 metres. The coupling in the circuit produced large effects bv its variation. - S. D-Carothers: Plane strain: the direct determination of stress. It is pointed out in the first part of the paper that in plane strain the stresses, if determined directly, are usually obtained by the aid of the well-known stress function method. The problem is usually that of finding a function y satisfying v,V=o throughout the bodv, with suitable values of x over the various boundaries. The most general value of x in Cartesian coordinates appears to be where A, B, C, and D are any constants and 8 is any plane harmonic function. It is shown that for any orthogonal co-ordinates the stresses derived by the stress-function method when applied to o^ = {x2 + y1)6 can always be resolved into two distinct sets, while in the case of Cartesian coordinates the stresses can be split up into four distinct sets. In view of the foregoing, the present paper has for its object the determination of the various sets of stresses which might legitimately occur in a state of plane strain, expressed in the simplest possible terms, with the view of rendering the building- up of a given state of stress a manageable operation. The paper sets forth the usual stress equations of equilibrium, and gives the identical relation between the strain components expressed in the various systems of coordinates. The various possible stress sets in rectangular and polar co-ordinates are then set forth in order, after which the solutions in orthogonal curvi-linears are obtained in such a manner as generally to show their connection with those formerly given. The second part of the paper applies the results obtained to the solution of some examples.-F. Horton and Ann C. Davies: An investigation of the effects of electron collisions with platinum and with hydrogen, to ascertain whether the production of ionisation from platinum is due to occluded hydrogen. The effects of bombarding a platinum surface by electrons the velocity of which could be gradually increased have been investigated by methods in which these effects were detected when superposed on the original electron current, and also when they were measured independently-It has been found that a genuine ionisation by electron impacts is produced at a minimum electron velocity of 13-0 volts, but that up to electron velocities of 30 volts no detectable amount of radiation capable of acting photo-electrically on platinum is obtained. In order to ascertain whether the ionisation produced at a minimum electron velocity of 13-0 volts arises from the platinum or from hydrogen attached to its surface, the effects of electron collisions with hydrogen were investigated in the same apparatus. A radiation was detected from this gas at a minimum electron velocity of 10-5 volts, and a second type of radiation at a minimum electron velocity of 13-9 volts. Three types of ionisation were also detected, beginning when the electrons acquired velocities of 13-0 volts, 14-4 volts, and 16-9 volts respectively. The first of these types is the ionisation obtained in a high vacuum, and experiments described in the paper show that this is not due to hydrogen, but arises from the platinum itself. From the experiments in hydrogen it is concluded that the minimum electron velocity for the production of radiation from a hydrogen atom is 10-5 volts, the minimum electron velocity for ionisation of the atom 14-4 volts, and the minimum electron velocity for ionisation of the molecule 16-9 volts. These results are in general agreement with the deductions from Bohr's theory. The second type of radiation, beginning at an electron velocity of 13-9 volts, is attributed to the hydrogen molecule.-L. Bairstow, R. H. Fowler, and D. R. Hartree: The pressure distribution on the head of a shell moving at high velocities. This paper describes a first attempt to measure the pressure distribution on a body moving through a gas at velocities equal to or greater than the velocity of sound a in the gas. The body in question is a spinning shell, movingr along its axis of symmetry, and the gas, air. The pressure at a given distance from the nose is communicated to a chamber in the head of the shell, and deduced from the time of burning a train of powder in this chamber, which is a quantity that can be directly observed. By a series of such observations the pressure at a given point of the head is determined as a function of the velocity ratio v/a, where v is the velocity of the shell relative to the air. Curves are obtained showing the variation of the pressure for values of v/a from 0-04 to 1-4, and for four different positions on the head of the shell.
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Societies and Academies. Nature 104, 617–619 (1920). https://doi.org/10.1038/104617a0
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DOI: https://doi.org/10.1038/104617a0