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Measurements concerning the electrical resistance of indium in the temperature field of liquid helium
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CURRENT TOPICS.
[J. F. I.
A Method of Distinguishing Natural Pearls from Those Artificially Grown. A. DAUVILLIER. (Comptes Rendus, Oct. 27, 1924.)--Direct examination with X-rays makes it possible to detect the pearl in the unopened oyster, but the observer cannot tell a good natural pearl from one grown by the Japanese procedure around a core of mother-of-pearl. The two pearls look alike, have the same chemical nature and possess almost the same density. The only sure way of distinguishing one from the other is to cut the pearl open and examine the structure. The author has tried to solve the problem by applying the method that has vielded~ such valuable results in the case of crystals. Monochromatic X-rays, the K-rays of rhodium or of silver, were sent through the pearls and the resultant figures were examined after several hours of exposure of the photographic plate. Good natural pearls showed a series of regular rings, sometimes sharply marked, while mother-of-pearl presented in every case Laue figures more or less regular and made up of patches more diffuse than in the case of crystals. These have a hexagonal symmetry when the rays are perpendicular to the cleavage planes, but are more numerous when the rays are parallel to the planes. Artificially grown pearls show a combination of the two figures just described, and can thus be distinguished from the natural gems. G.F.S. Measurements Concerning the Electrical Resistance of I n d i u m in the Temperature Field of Liquid Helium. W. T u r N and H. KAMERLINGH ONNES. (Communications, Phys. Lab., Univ. of Leyden, No. I 6 7 . ) - - " F o r the further detection of supraconducting metals it seems desirable to investigate the behavior of those elements which take a place near already known supraconductors in the periodic system. Indium--above thallium and by the side of tin--seemed a suitable metal." Accordingly from chemically pure indium four coils of wire were prepared with resistances ranging from 3.7 to 4.7 ohms. These were placed in a helium bath, the temperature of which was determined by measuring its vapor pressure. At a temperature close to 3.4 ° abs. all the coils possessed no resistance that could be detected. For the three best coils the resistance at 3.42o abs. was about I/3O of the resistance at ordinary temperature. G.F.S. The Constant a of the S t e f a n - B o l t z m a n n Law. A. KUSSMANN.
(Zeit. f. Physik, Vol. 25, No. ~.)----The radiation in watts from I sq. cm. of a black body in a vacuum is calculated by multiplying the constant a by the fourth power of the temperature of the radiating area. Since I898 twenty determinations have been published with results ranging from 5.3 to 6.5 × IO-12 watt/cm2. In this paper a careful redetermination is made and the final value presented is 5.79. This lies between the result of Coblentz and Emerson in Washington, 5.72, and that of Gerlach in I912 , 5.8o. G.F.S.
CURRENT TOPICS.
[J. F. I.
A Method of Distinguishing Natural Pearls from Those Artificially Grown. A. DAUVILLIER. (Comptes Rendus, Oct. 27, 1924.)--Direct examination with X-rays makes it possible to detect the pearl in the unopened oyster, but the observer cannot tell a good natural pearl from one grown by the Japanese procedure around a core of mother-of-pearl. The two pearls look alike, have the same chemical nature and possess almost the same density. The only sure way of distinguishing one from the other is to cut the pearl open and examine the structure. The author has tried to solve the problem by applying the method that has vielded~ such valuable results in the case of crystals. Monochromatic X-rays, the K-rays of rhodium or of silver, were sent through the pearls and the resultant figures were examined after several hours of exposure of the photographic plate. Good natural pearls showed a series of regular rings, sometimes sharply marked, while mother-of-pearl presented in every case Laue figures more or less regular and made up of patches more diffuse than in the case of crystals. These have a hexagonal symmetry when the rays are perpendicular to the cleavage planes, but are more numerous when the rays are parallel to the planes. Artificially grown pearls show a combination of the two figures just described, and can thus be distinguished from the natural gems. G.F.S. Measurements Concerning the Electrical Resistance of I n d i u m in the Temperature Field of Liquid Helium. W. T u r N and H. KAMERLINGH ONNES. (Communications, Phys. Lab., Univ. of Leyden, No. I 6 7 . ) - - " F o r the further detection of supraconducting metals it seems desirable to investigate the behavior of those elements which take a place near already known supraconductors in the periodic system. Indium--above thallium and by the side of tin--seemed a suitable metal." Accordingly from chemically pure indium four coils of wire were prepared with resistances ranging from 3.7 to 4.7 ohms. These were placed in a helium bath, the temperature of which was determined by measuring its vapor pressure. At a temperature close to 3.4 ° abs. all the coils possessed no resistance that could be detected. For the three best coils the resistance at 3.42o abs. was about I/3O of the resistance at ordinary temperature. G.F.S. The Constant a of the S t e f a n - B o l t z m a n n Law. A. KUSSMANN.
(Zeit. f. Physik, Vol. 25, No. ~.)----The radiation in watts from I sq. cm. of a black body in a vacuum is calculated by multiplying the constant a by the fourth power of the temperature of the radiating area. Since I898 twenty determinations have been published with results ranging from 5.3 to 6.5 × IO-12 watt/cm2. In this paper a careful redetermination is made and the final value presented is 5.79. This lies between the result of Coblentz and Emerson in Washington, 5.72, and that of Gerlach in I912 , 5.8o. G.F.S.