The invention itself is beautifully simple: you just press a card against a spiked wheel.* Turn the wheel slowly and you’ll hear distinct clicks as the card hits each tooth, but turn it faster and those clicks blur into a tone. The faster the wheel turns, the higher the tone gets. You’ve already invented a Hooke’s wheel if you ever stuck a card into the spokes of your bicycle (Section 10.12.1), and by spinning your Hooke’s wheel such that the card gets hit 440 times per second, the sound you produce will be A440.
