A device (Fig. 1) called a “gear instrument” was introduced in a book titled The World of Sound by William Henry Bragg in the early 20th century.1 Made up of large and small gears and a motor, the device is played by placing a piece of thick paper on the rotating gear teeth. The sound produced is reminiscent of the sound of a trumpet or a violin. When demonstrated in the classroom, students recognize that the sound comes out in the order of “do,” “re,” “mi,” as the paper is moved from the small gears to the large gears. Students further learn that the ratio of the number of the teeth of gears is the same as the ratio of frequencies of the musical scale, 24:27:30:32:36:40:45:48.2 

Unfortunately, the gear instrument is not found in most schools due to the machining required in its production. The device shown...

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