Appunn’s “tenor” tonometer
(The following description is from a “Description of Appunn’s Tonnemeters,” by Alexander J. Ellis, F.R.S., &c.; MS in the Western Galleries of the South Kensington Museum, 1880. With the tonometers. Science Museum technical file T/1876-466. The note was written by Alexander J. Ellis, who acquired the apparatus for the South Kensington Museum’s collection in 1876, and later edited either by Ellis himself or a museum keeper. The accompanying tables mentioned at the end of the note are missing from the museum’s files.)
Appunn’s “Tenor” Tonometer
Made by George Appunn and Sons
This instrument is designed to give an accurate indication by means of beats of the pitch or vibration frequency of any note within its limits of action which is sounded in its neighbourhood.
The instrument consists of a number of harmonium reeds giving 65 tones, each of which is higher by 4 vibrations than the previous one, from c’ (256) to c’’ (512). The reeds are screwed to a perforated board forming the bottom of an oblong box in the upper part of the apparatus; this box forms part of the bellows by means of which the reeds are actuated.
Each reed is placed over a perforation in the box, which is externally closed by a valve; the valve communicates by a wire to a knob or pull outside, which is numbered, the lowest being (0) and the highest (64). The reeds are so tuned that each beats exactly four times a second with either of the adjacent reeds, but by pushing in the valve so that the perforation is partially shaded, the note is flattened up to about 21/2 vibrations per second, p. 102. This forms an important and valuable feature of the tonometer.
The note given by a harmonium reed is rich in upper partials, and so is very complex. This reed quality is of great service, because it perfectly distinguishes all the consonances, allowing a slight error to be immediately detected by dissident beats.
To find the pitch of any sound, the reeds of the tonometer are tried against it until two are found which give beats with it not exceeding four a second. The frequency of the fork will be higher than one, and lower than the other.
By carefully counting the beats in a measured time, and making corrections according to the tables accompanying the instrument, very accurate results may be obtained.