Abstract
EDINBURGH
Royal Society, June 21.—Prof. Maclagan, M.D., vice-president, in the chair.—Prof. Chrystal read a paper on a differential telephone, and on the application of the telephone to electrical measurements. A differential telephone was exhibited. It differed from an ordinary telephone in much the same way that a differential galvanometer differs from an ordinary single-coiled one. Two thin wires were twisted together and wound round the magnet in the usual way. It was shown that when an interrupted current passed in opposite directions through the two coils of the differential telephone no sound was heard. In using the instrument, its two coils were put into the two branches of a multiple arc, which was inserted in the circuit of the interrupted current. The interrupted currents of the two branches passed in opposite directions through the coils. The conditions for perfect compensation were not only that the resistances of the two branches must be equal, but also that their co-efficients of self-induction must be the same. If only one of these conditions was fulfilled a minimum of sound could be got, but absolute silence was impossible. The necessity for this twofold adjustment had not been hitherto sufficiently recognised; and it was to its neglect that the main difficulties in using Hughes induction-balance were no doubt to be referred. Some years ago Prof. Chrystal had worked out the mathematics of the subject, but had been unable till recently to corroborate his results by experiment. Prof. Chrystal then proceeded to indicate how such a differential telephone could be applied to the measurement of coefficients of self-induction in terms of an arbitrary unit. Two coils were prepared of exactly the same resistance, but one was so wound as to have practically no self-induction. The self-induction of the other was the arbitrary unit mentioned above. In the rough model shown, two coils, whose distance apart could be varied at will, were introduced into each branch of the multiple arc above referred to, and were first adjusted so as to produce perfect compensation in the differential telephone. The other two equal resistance-coils were then introduced, one into the circuit of each induction pair, with the necessary effect of destroying the compensation. By a readjustment of the induction of one of the pairs, compensation was again secured, the change of distance of the coils of the altered pair corresponding therefore to the arbitrary unit. The two single coils were then removed, a fresh compensation obtained by alteration of the other induction pair, the single coils again introduced, a fourth compensation effected and a second stage reached in the formation of a graduated scale of self-induction in terms of an arbitrary unit; and so on till a complete scale was formed. Prof. Chrystal further pointed out how his instrument might be used for measuring capacities, and for investigating the real nature of the opposition offered by electrolytes to the passage of electric currents.—Prof. Tait communicated a paper on the determination of the specific heats of saline solutions, by Mr. Thomas Gray, B.Sc.—Mr. J. Y. Buchanan described a "navigational sounding-machine"of very simple construction. A glass tube, closed below by a plug kept sufficiently tight by a close-fitting india-rubber band, was provided above with a peculiarly-formed capillary orifice. The tube was first allowed to fill with air, and then sunk to the required depth in the sea. The air was compressed under the increased pressure, and the water began to trickle in from above. The quantity of water which so gained admittance was the datum from which the pressure, and therefore the depth, could be calculated. The water was removed by taking away the bottom plug; and the instrument was once more in a state suitable for use. Mr. Buchanan also communicated some experiments on the compressibility of glass. The value he obtained was greater than that obtained by Grassi by 21/2 per cent.—Dr. Macfarlane read a short paper entitled “uggestions on the Art of Signalling.” He advocated the use of three qualities or symbols in preference to the dot-and-dash or two-symbolled alphabet of Morse, arguing that such a system would be found more rapid than the latter.—Dr. R. M. Ferguson communicated a note on the wire telephone, following up tlie results obtained formerly by himself and those more recently arrived at by Preece and Chrystal. He showed that the sound emitted by a stretched iron wire through which an interrupted current was passing varied [in a remarkable way with temperature, reaching a most evident maximum about a dull red heat. This variation he regarded as being in some way connected with the magnetic properties of iron, and on that ground criticised Prof. Chrystal's explanation of the De la Rive phenomenon as being due to rapid contractions and dilatations of the thin wires through which the current passed. In the remarks which followed Prof. Chrystal admitted the influence of magnetism in the case of the iron, a thick wire of which was as efficient as a thin wire; but in the case of what are usually reckoned non-magnetic metals, only thin wires of which are efficient for reproducing continuous sounds, he still thought that the true explanation was to be found in their changes of length. The altogether peculiar action of iron—though probably nickel and cobalt would have a similar action—seemed to himrather to favour this view than the other.
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Societies and Academies . Nature 22, 331–332 (1880). https://doi.org/10.1038/022331a0
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DOI: https://doi.org/10.1038/022331a0