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The sonometer

This instrument was devised to verify the laws of the vibrating string proposed by Mersenne in 1636.

He stated that the frequency of a stretched string was:
(a) proportional to the square root of the tension,
(b) inversely proportional to the length of the string, and
(c) inversely proportional to the square root of the mass per unit length of the string.

The sonometer itself consists of a wire stretched between two supports or bridges on a wooden sounding box. The tension of the string may be varied either by a screw or by hanging weights on one end of it. The length of the vibrating wire can be changed using a small wooden support that can be moved along under the wire.

The purpose of the sounding box is to make a larger mass of air vibrate and so amplify the very small sounds produced by the vibrating string itself. The same principle is applied in stringed instruments such as the guitar or cello.

The frequency at which the string is vibrating can be found by using a tuning fork of known frequency, which is made to vibrate and held with its base on the sounding box. The string is then tuned to give the same note. A small piece of paper may be placed on the centre of the wire and when the frequency of the wire is tuned to that of the fork resonance occurs and the wire will vibrate, throwing the piece of paper off. The tension, length and mass per unit length of the string can then easily be measured.


© Keith Gibbs