This consists of a graduated arc, a pendulum with a hard rubber bob, and a block of ebony, so designed that the intensity of the sound produced, when the bob hits the block, is proportional to the height of fall of the bob. The first device of this sort was made by Alfred W. Volkmann (1800-1877), a German physiologist.
Ref: C. H. Stoelting Co., Psychological and Physiological Apparatus and Supplies (Chicago, 1930), p. 37.
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