The reason for using wave machines in demonstration lectures is the same today as in the 19th century when the transverse machine at the left was designed: students have trouble visualizing complex and changing phenomena. This machine was made by Ritchie of Boston and appears in his 1860 catalogue as "Snell's Illustration of Water Waves" at a cost of $20.00.     The machine is in regular use at Kenyon College in Ohio.

The mechanism of the transverse wave machine is shown at the left. Eccentric cams are fastened with set-screws onto a long shaft turned with the crank on the end. Each identical cam is rotated thirty degrees with respect to its immediate neighbor. The sheet-metal sliders rest on top of the cams, and move up and down as the crank is turned.    In demonstrations the waveshapes may be made to move to the right or to the left by reversing the direction of rotation of the crank.

   The transverse wave machine below appears in the 1912 catalogue of the C. H. Stoelting Co. of Chicago. The catalogue copy reads: Wave Apparatus. This is a simple but very perfect device to illustrate wave motion, and is decidedly superior to most similar devices, as it starts with all the particles at rest in a straight line. By slowly turning the crank the successive vertical motions of the white balls combine to produce a perfect onward wave. By turning the handle backward a couple of turns the balls will again fall into a straight line as at first. The apparatus is large enough so that the motion may be plainly observed by all in a large hall ... $9.00" This is from the collection of Ron Watson, and the photographs are by Adrax Advertising.

Since this is a small piece of apparatus, it seems only fair to give it a large picture! The 1900 catalogue published by Max Kohl of Chemnitz lists this as a wave machine for the projection lantern at a cost of about $12. The device clearly does not fit into the gate of a projector, but is used instead for shadow projection, a technique more common in Germany than in the United States.    This particular example is at the Smithsonian Institution (catalogue number 325,987). Other examples survive (and are in use) at the University of Notre Dame, and Knox College in Galesburg, Illinois.

The large wave machine below was bought in the late 1920s by the physics department of Hobart and William Smith Colleges in Geneva, New York. It was used for demonstrating both transverse and longitudinal waves, and was made by the firm of Max Kohl of Chemnitz, Germany.