Interventions to increase the participation of women in physics     

Dresselhaus MS  Franz JR  Clark BC 《Science (New York, N.Y.)》1994,263(5152):1392-1393

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Very-Long-Baseline Radio Interferometry: The Mark III System for Geodesy, Astrometry, and Aperture Synthesis     

Rogers AE  Cappallo RJ  Hinteregger HF  Levine JI  Nesman EF  Webber JC  Whitney AR  Clark TA  Ma C  Ryan J  Corey BE  Counselman CC  Herring TA  Shapiro II  Knight CA  Shaffer DB  Vandenberg NR  Lacasse R  Mauzy R  Rayhrer B  Schupler BR  Pigg JC 《Science (New York, N.Y.)》1983,219(4580):51-54

The Mark III very-long-baseline interferometry (VLBI) system allows recording and later processing of up to 112 megabits per second from each radio telescope of an interferometer array. For astrometric and geodetic measurements, signals from two radio-frequency bands (2.2 to 2.3 and 8.2 to 8.6 gigahertz) are sampled and recorded simultaneously at all antenna sites. From these dual-band recordings the relative group delays of signals arriving at each pair of sites can be corrected for the contributions due to the ionosphere. For many radio sources for which the signals are sufficiently intense, these group delays can be determined with uncertainties under 50 picoseconds. Relative positions of widely separated antennas and celestial coordinates of radio sources have been determined from such measurements with 1 standard deviation uncertainties of about 5 centimeters and 3 milliseconds of arc, respectively. Sample results are given for the lengths of baselines between three antennas in the United States and three in Europe as well as for the arc lengths between the positions of six extragalactic radio sources. There is no significant evidence of change in any of these quantities. For mapping the brightness distribution of such compact radio sources, signals of a given polarization, or of pairs of orthogonal polarizations, can be recorded in up to 28 contiguous bands each nearly 2 megahertz wide. The ability to record large bandwidths and to link together many large radio telescopes allows detection and study of compact sources with flux densities under 1 millijansky.  相似文献   

EVOLUTION     

Clark AH 《Science (New York, N.Y.)》1929,69(1784):271-272

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Contraceptives and dysplasia: higher rate for pill choosers     

E Stern  V A Clark  C F Coffelt 《Science (New York, N.Y.)》1970,169(944):497-498

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MICROORGANISMS AND VITAMIN PRODUCTION IN GREEN PLANTS     

Clark NA  Thomas BH 《Science (New York, N.Y.)》1934,79(2060):571-572

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Calcification on an Unstable Substrate: Marginal Growth in the Moilusk Pecten diegensis     

Clark GR 《Science (New York, N.Y.)》1974,183(4128):968-970

Observations of the growing margin of Pecten diegensis show that calcification can occur on an unstable substrate, although this initial shell material is quite disordered. Later growth on the inner surface of the disordered material soon becomes ordered, which suggests that the calcification process benefits from the stability of a mineralized substrate, but does not utilize the substrate as a physical or stereochemical template.  相似文献   

Two G protein oncogenes in human endocrine tumors   总被引：55，自引：0，他引：55  

J Lyons  C A Landis  G Harsh  L Vallar  K Grünewald  H Feichtinger  Q Y Duh  O H Clark  E Kawasaki  H R Bourne 《Science (New York, N.Y.)》1990,249(4969):655-659

Somatic mutations in a subset of growth hormone (GH)-secreting pituitary tumors convert the gene for the alpha polypeptide chain (alpha s) of Gs into a putative oncogene, termed gsp. These mutations, which activate alpha s by inhibiting its guanosine triphosphatase (GTPase) activity, are found in codons for either of two amino acids, each of which is completely conserved in all known G protein alpha chains. The likelihood that similar mutations would activate other G proteins prompted a survey of human tumors for mutations that replace either of these two amino acids in other G protein alpha chain genes. The first gene so far tested, which encodes the alpha chain of Gi2, showed mutations that replaced arginine-179 with either cysteine or histidine in 3 of 11 tumors of the adrenal cortex and 3 of 10 endocrine tumors of the ovary. The mutant alpha i2 gene is a putative oncogene, referred to as gip2. In addition, gsp mutations were found in 18 of 42 GH-secreting pituitary tumors and in an autonomously functioning thyroid adenoma. These findings suggest that human tumors may harbor oncogenic mutations in various G protein alpha chain genes.  相似文献   

Environmental nitroso compounds: reaction of nitrite with creatine and creatinine   总被引：1，自引：0，他引：1  

M C Archer  S D Clark  J E Thilly  S R Tannenbaum 《Science (New York, N.Y.)》1971,174(16):1341-1343

Creatine reacts with nitrite under acid conditions to produce first sarcosine and then N-nitrososarcosine, which is a weak carcinogen in the rat. Creatinine reacts with acidified nitrite to produce either creatinine-5-oxime or 1-methylhydantoin-5-oxime, depending on reaction conditions. The toxicity and environmental significance of these compounds is not yet known.  相似文献   

Nimbus-7 coastal zone color scanner: system description and initial imagery     

Hovis WA  Clark DK  Anderson F  Austin RW  Wilson WH  Baker ET  Ball D  Gordon HR  Mueller JL  El-Sayed SZ  Sturm B  Wrigley RC  Yentsch CS 《Science (New York, N.Y.)》1980,210(4465):60-63

The Coastal Zone Color Scanner (CZCS) on Nimbus-7, launched in October 1978, is the only sensor in orbit that is specifically designed to study living marine resources. The initial imagery confirms that CZCS data can be processed to a level that reveals subtle variations in the concentration of phytoplankton pigments. This development has potential applications for the study of large-scale patchiness in phytoplankton distributions, the evolution of spring blooms, water mass boundaries, and mesoscale circulation patterns.  相似文献   

Interferometer experiment with independent local oscillators   总被引：1，自引：0，他引：1  

Bare C  Clark BG  Kellermann KI  Cohen MH  Jauncey DL 《Science (New York, N.Y.)》1967,157(3785):189-191

We have operated an interferometer with independent local oscillators and without any communication link of wide bandwidth between the elements of the interferometer. This makes operation possible at very long base lines because, heretofore, construction of the communications link has been the factor limiting the separation of the elements. In our system, coherence at the two elements is maintained through the use of two highly stable, atomic oscillators. The intermediate-frequency output signals are recorded at each element on a high-speed digital tape recorder. Interference fringes are produced later by cross-correlating the two tape records in a digital computer.  相似文献