This manometric flame interference apparatus is part of the large collection of Koenig apparatus at the University of Toronto. It is catalogue number 243 in the 1889 Koenig catalogue, and cost 250 francs (about $50). Similar apparatus was made by Max Kohl.
The signal produced by a tuning fork is concentrated with a Helmholtz resonator and fed into the two tubes with the inverted U shape. One tube has a fixed length and the other has a variable length, calibrated in millimeters. Each tube ends in a manometric flame, a gas flame whose height indicates the intensity of the acoustic signal delivered to it. In addition, the output of each tube is fed together to a third flame. The adjustable tube is moved in or out until the two signals are in phase, indicated by a large amplitude for the third flame. Its path length is then increased until a minimum is obtained, then a maximum, etc. Every time the acoustical path length is increased by one wavelength, the manometric flame height goes through a complete cycle. Meanwhile, the flames attached to the individual tubes remain at the same height.
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