Humboldt State University
Robert A. Paselk Scientific Instrument Museum
Spring Balance Universal Sonometer
W.M. Welch Scientific Company
Provenance: Humboldt S-T-C; c.1930
Used to demonstrate the "laws of vibrating strings" in physics laboratory classes. In this law the relationship of the length of the string (in simplest vibrational mode= one-half the wavelength) the tension on the string in dynes, and the mass/unit length of the string, to the frequency of vibration of a string is described. In use the length of the string is adjusted by moving the bridge until it vibrates in unison with a tuning fork of known frequency. The strings are sounded by striking with a small rubber mallet.
The body of the instrument is about 45" long and 5" wide, made of steel angle-iron rails bolted to cast iron open-work end-pieces. There is a 12" long sheet-steel sound box at the tail end with a black stained hardwood bridge located with brass pins and held in place by the strings of the instrument 20 cm before the opening of the sound-box. The frame and sound-box are all painted in gray wrinkle-finish enamel. A white-enameled steel sheet begins at the opening of the sound-box and extends to end of the instrument. A central scale runs from 20100 cm by 0.1 cm with each cm graduation numbered. Diatonic and equally tempered scales are also marked, from C512C128. At the top end off the instrument a cast-iron and steel rack extends another 9" and holding three adjustable spring scales with hooks to tension the three strings. Each scale is graduated (red filled engraving) from 020 units by 0.25 units, with even graduations numbered. The scales themselves are of bras with white-metal plating. Each scale is attached via a square screw with wing-nut for adjusting tension to the cast iron rack. With the springs fully tensioned the entire instrument would have a total length of 57 1/2". The original adjustable bridge of the instrument is missing: a custom wooden bridge has been substituted. The instrument is in excellent condition barring a pattern of pitting of the white paint on the scale with consequent corrosion, and a chip on the cast-iron of the scale rack.
There is a brass Humboldt S-T-C tag (4857) riveted to the upper left end of the scale.
Minor, Ralph S. Physical Measurements; A Laboratory Manual in General Physics for Colleges. Associated Student's Store, Berkeley (1947) Part II, pp. 94-97