Science and music / [by Sir James Jeans].

  • Jeans, James, 1877-1946.
Date:
1947
Catalogue details

Licence: In copyright

Credit: Science and music / [by Sir James Jeans]. Source: Wellcome Collection.

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    cork filings or lycopodium powder. When the tuning-fork is set into vibration, the particles of powder exhibit the agitation of the air by dancing about, so that the whole tube is filled with dust. When the stopper reaches a position in which resonance occurs, the agitation becomes even more violent throughout most of the tube, but there are certain points of calm, at which the powder begins to settle down, and finally forms little piles. In the instance just taken, we should find that these are at distances of Fig. 42. If we put dust into the vibrating column of air, this settles in little piles at evenly spaced points A, B, C ..., shewing thiat these points are nodes of the vibration. i foot, 2 feet, 3 feet, and so on, from T, and as the length from S to T is an exact number of feet, the last heap of all will be close up against S, as in fig. 42. It is easy to see what has happened. As the dust can stay at rest at these particular points, it is clear that there can be no agitation of the air there; these points must then be “nodes’5 (p. 75) of the vibration. In other words, the equal lengths of air TA, ABy BC, CS in fig. 42 must be vibrating separately—-just as the stretched string vibrated in separate equal lengths (p. 67).* Thus the free vibrations * It may seem paradoxical that T is a node, although the loose stopper is moving backwards and forwards, keeping the air in motion there. The explanation is that the stopper has a far greater weight than all the air in the tube, so that a quite small motion of the stopper imparts enough energy to the air to keep it in violent agitation. Thus the amplitude of the stopper is small compared with that at other parts of the tube, and T is virtually a node, or very nearly so.