Hopkins' Divided Tube
    The forked acoustic tube, invented by William Hopkins (1793-1866) about 1835, is used to detect the relative phasing of adjacent sections in a Chladni Plate. As one of these sections is moving upward, creating a compression in the air above it, the other is moving downward, producing a rarefaction. The open legs of the tube are placed over the adjacent segments, and sample the signals. Since they are 180 degrees out of phase, the signals cancel, and no sound comes from the tube. If you cover one of the legs with your hand, the sound will be clearly audible. The sound will be louder if the two legs are positioned over two segments of the Chladni plate which are in phase.
    The wooden tube from Denison University was probably made by an undergraduate in the eighteen nineties. The metal tubes are listed at $3.15 in the 1916 catalogue of the Knott Apparatus Company of Boston. The telescoping section is not used for resonance effects, but enables the lower openings to be positioned close to the surface of the Chladni plate.

REFERENCE: Thomas B. Greenslade, Jr., "Nineteenth Century Textbook Illustrations XXIX: Hopkins' Divided Tube", Phys. Teach., 17, 113-114 (1979)

                              Denison University
                             Colby College                                                                   Smithsonian Institution

                 The College of Wooster                                                  University of Mississippi

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