Observe a demonstration of a Projected Wave Machine from the Physical Science Collection at the Smithsonian's National Museum of American History.
This apparatus consists of three wires, each bent to resemble transverse waves. The wires are mounted in a rectangular brass box that was placed in front of the lens of a projector. The top two wires are identical, but positioned so that their shadows appear to move in opposite directions as a crank on the side of the box is turned. As they do this, the crests and troughs of the waves alternately lineup and overlap. The corresponding "interference" of the two waves is seen in the changing shape of the third wire. The sliding covers on the box allowed the instructor to keep the demonstration hidden until the rest of the presentation had been completed. Although it is unmarked, this apparatus bears a strong resemblance to wave models made in the early 20th century by the William Gaertner Company of Chicago.
