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5 either one-half or one centimeter apart and have vulcanite knobs upon which to rest the fingers. As soon as the approximate reading is found with one key the'block is moved until both keys when pressed give a tone of fhe same intensity ; the middle point is then the true minimum. *It is well to have the one key movable so that the distance between the contacts may be reduced as the point is reached. By the use of this arrangement it is possible to find the point within a small fraction of adoi:10.1021/ja02039a002 fatcat:u5x2asllwvfmhcbgqz2wptiig4