Abstract
LONDON Physical Society, June 26.— Prof. W. E. Ayrton, F.R.S., Vice-President, in the chair.—Mr. E. M. Langley was elected a Member of the Society..—The following communications were read:.—On certain sources of error in connection with experiments on torsional vibrations, by Mr. Herbert Tomlinson. During a long series of researches on the torsional elasticity and internal friction of metals, the author has come across the following sources of error in connection with torsional vibrations. In some of the earlier experiments a horizontal brass bar was suspended by a wire and oscillated, the times of oscillation being observed by the ordinary lamp, mirror, and scale. The moment of inertia was varied by sliding two brass cylinders, suspended from the bar by fine wires, backwards and forwards along it. It was then found that under certain conditions the bar executed a few vibrations of rapidly decreasing amplitude, came to rest, and then commenced to swing again, the amplitude increasing to a maximum, again decreasing, and so on. This effect was finally traced to an approach to synchronism between the time of oscillation of the bar and that of the small cylinders about their axes of suspension, the absorption of energy being due to these being set in vibration. The effect entirely disappeared upon clamping the cylinders rapidly to the bar. On another occasion, however, the old phenomenon reappeared, and after much time spent in investigating it, was found to be due to a somewhat similar cause, a near approach to synchronism between the periods of torsional and pendulous vibrations. If the axis of the wire passed accurately through the centre of mass of the vibrator, this would not occur; and this condition it is practically impossible to fulfil. Another source of error lies in the fact that, in a wire recently suspended, the torsional vibration-period will always be found to be slightly greater than when it has been suspended for some time and frequently oscillated. - On a mode of driving electric tuning-forks, by Prof. S. P. Thompson. It is invariably found that the frequency of an electrically maintained fork is continually changing. This great inconvenience the author believes to be due to the fact that the impulses are given to the prongs at a disadvantageous moment, namely, when they are at the extremities of their swings. It is desirable that the impulse should be given at the middle of the swing, and to effect this it is suggested that each fork should make and break the circuit of the magnet influencing the other one, and it was shown how the electrical connections could be made to effect this in a simple manner.—Prof. Silvanus P. Thompson then read some further notes on the formulae of the electro -magnet and of the dynamo. The author pointed out that a misapprehension of his former paper on this subject had given rise to certain critical remarks by Dr. O. Fröhlich, to which he replied. The author also explained the new form given recently by Dr. Fröhlich to the formula of the electro-magnet, rendering it much more readily applicable to the various equations of dynamo-machines. Formerly the Lamont-Fröhlich formula had been written —
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Societies and Academies . Nature 34, 283–284 (1886). https://doi.org/10.1038/034283a0
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DOI: https://doi.org/10.1038/034283a0