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fork in fig. 39, it will be seen from what has been said above, that this fork also will be set in vibration. Further, the current before returning to the battery may be led through several such electro­magnets, furnished with tuning-forks; and if the latter have vibra­tion numbers which are the same, or any multiples of that of the fork in fig. 40, they will be kept in vibration also. Now, the vibration numbers of overtones are multiples of the vibration number of the fundamental, and therefore only one such fork as that of fig. 40 is necessary, in exciting any number of forks such as that of fig. 39, if the latter are tuned to the series of partials and the former is in unison with the fundamental.
Fig. 41 shows the method by which Helmholtz obtained variations in intensity in his apparatus. D represents one of the tuning-
Fig. 41.
forks, kept in vibration by an electro-magnet, which is not shown in the figure. A is a resonator of suitable dimensions, the aperture of which can be closed by the cap, 0. When thus closed the sound of the fork is almost inaudible, but, on gradually opening the aperture, the sound comes out with increasing loudness; the maximum being reached when the aperture is quite uncovered. In the figure, the resonator is shown, for the sake of distinctness, at a distaLce from the fork; when in use it may be pushed up as close to the fork as desired, by means of the stand working in the groove below. The cap covering the aperture of the resonator is connected, by means of levers and wires as seen in the figure, with