HANDBOOK OF ACOUSTICS - online book

A complete view of Acoustical Science & its bearings on music, for musicians & music students.

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ON THE PITCH OF MUSICAL SOUNDS.           35
in the figure, and is stretched by a weight at the other. The sound is evolved by drawing a violin bow over the wire. In order to find the vibrational number of any given sound, the wire is first tuned in unison with it, by varying the length of the wire (by means of the movable bridge shown in fig. 23) or the stretching weight. This weight is then noted, and the vibrating part of the wire measured and weighed. If the number of grains in the stretch­ing weight (including the weight of the adjacent non-vibrating part of the wire) be denoted by W; the number of inches in the vibrating wire by L ; and the number of grains in the same by w; then the vibrational number is
P being the length of the seconds pendulum at the place of obser­vation, at Greenwich), and tr the constant 3-14159. Although theoretically perfect, the practical difficulties of determin­ing the unison, measuring the length, ascertaining the weights, obtaining perfect uniformity in the wire, keeping the temperature constant, together with those arising from the thickness of the wire, are so great, as to render this method of no avail where accuracy is required.
A far more accurate method of counting vibrations is that known as the Graphic method, which is, however, only applicable in general to tuning-forks. The principle of this method will be readily understood on reference to fig. 24. A light style is attached to one of the prongs of a tuning fork. A piece of paper or glass, which
Fig. 24.
has been coated with lamp-black by holding it in the smoke of burning camphor or turpentine, is placed below the tuning fork so that the style just touches it. If now, while the glass remains at rest, the fork be set in vibration, the style will trace a straight bine on the glass or paper, by removing the lamp black in its path as it moves to and fro. But if the glass be moved rapidly and steadily