Proceedings of the American Academy of Arts and Sciences Volume 15 - Softcover

This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1880 Excerpt: ...also a convenient method of studying wave motion on the surface of mercury. The method of Niemoller consists in providing a stretched iron wire, through which a current flows, with a platinum point at its middle, which dips in mercury, and placing a magnet above and near the middle of the half of the wire through which the current flows. The wire, once set in vibration, is maintained in motion by the attraction of the magnet. I substituted a large shallow evaporating dish filled with clean mercury for the small connecting cup used by Niemoller, and an electro-magnet for his permanent magnet. Since the electro-magnet consisted of a number of coarse wire coils, and formed part of the circuit with the iron wire, a bright spark was produced at the break, on the surface of the mercury. The mercury was then covered with an extremely fine layer of lycopodium dust, which was effected by blowing off the lycopodium after it had been sifted over the mercury. When the wire was set in vibration, beautiful rings, very similar in appearance to Newton's rings, emanated from the point where the break was made, and spread out over the surface of the mercury. These rings were almost as sharply defined as Newton's rings, and were due to the illumination of the waves by the reflection of the light of the electric spark from the fine particles of lycopodium dust. The regularity of the rings of light made it possible to measure their radii, and in this way determine the rate of vibration of different lengths of wires. The following numbers represent one of several measurements which I made on wire of the same diameter:--Wire 100 cm. in length 2 waves to 5 mm. "50 cm." 4 "" Thus the wave lengths were found to be proportional to the lengths of the wire. The inte...