Étude des grandeurs caractéristiques de la tache cathodique d'un arc électrique

The cathode of an electric arc burning freely in air was studied during a half-wave of a sinusoidal, 50 Hz current. For mean intensities ranging from 500 to 2 000 A, five characteristic parameters for the cathode spot were determined : the mean radius a, the temperature T, the electric field E, the total current density J, and the fraction s of the current density due to electrons. The experimental measurement of the erosion of the cathode together with five simultaneous equations enabled the parameters above to be obtained for copper and silver electrodes. The first part of the results gives mean values. The second part shows the changes occuring in the cathode spot as a function of time for the half-wave of the current.On étudie la cathode d'un arc électrique brulant librement dans l'air pendant une demi-alternance d'un courant sinusoïdal de fréquence 50 Hz. Nous avons déterminé pour des intensités moyennes variant entre 500 et 2 000 A cinq grandeurs physiques caractéristiques de la tache cathodique : le rayon moyen a, la température T, le champ électrique E, la densité totale de courant J et la fonction s de la densité de courant due aux électrons. La mesure expérimentale de l'érosion de la cathode associée à un système de cinq équations a permis d'obtenir ces grandeurs pour des électrodes en cuivre et en argent. La première partie des résultats donne des valeurs moyennes, tandis que la deuxième partie montre l'évolution de la tache cathodique en fonction du temps pour la demi-alternance de courant

Are inter-model differences in AMIP-II near surface air temperature means and extremes explained by land surface energy balance complexity?

We use the Atmospheric Model Intercomparison Project (AMIP-II) experimental protocol to perform six ensemble simulations for four different modes of land surface energy balance complexity. We calculate the land-only mean, minimum and maximum temperature variance for each ensemble and compare these with AMIP-II results. We find no evidence that the surface energy balance complexity leads to systematic differences in the simulated mean, minimum or maximum temperature variance at the global scale, or in the zonal averages. We therefore conclude that the differences simulated by the AMIP-II models cannot be attributed to variations in surface energy balance complexity. Our results indicate that the simulation of mean temperature variance and the variance of extreme temperature are not limited by uncertainties in how to parameterize the surface energy balance, a conclusion that adds confidence to the use of climate models for detection and attribution studies.4 page(s

Cytotoxic activity of polyacetylenes and polyenes isolated from roots of Echinacea pallida

BACKGROUND AND PURPOSE: The n-hexane extracts of the roots of three medicinally used Echinacea species exhibited cytotoxic activity on human cancer cell lines, with Echinacea pallida found to be the most cytotoxic. Acetylenes are present in E. pallida lipophilic extracts but essentially absent in extracts from the other two species. In the present study, the cytotoxic effects of five compounds, two polyacetylenes (namely, 8-hydroxy-pentadeca-(9E)-ene-11,13-diyn-2-one (1) and pentadeca-(9E)-ene-11,13-diyne-2,8-dione (3)) and three polyenes (namely, 8-hydroxy-pentadeca-(9E,13Z)-dien-11-yn-2-one (2), pentadeca-(9E,13Z)-dien-11-yne-2,8-dione (4) and pentadeca-(8Z,13Z)-dien-11-yn-2-one (5)), isolated from the n-hexane extract of E. pallida roots by bioassay-guided fractionation, were investigated and the potential bioavailability of these compounds in the extract was studied. EXPERIMENTAL APPROACH: Cytotoxic effects were assessed on human pancreatic MIA PaCa-2 and colonic COLO320 cancer cell lines. Cell viability was evaluated by the WST-1 assay and apoptotic cell death by the cytosolic internucleosomal DNA enrichment and the caspase 3/7 activity tests. Caco-2 cell monolayers were used to assess the potential bioavailability of the acetylenes. KEY RESULTS: The five compounds exhibited concentration-dependent cytotoxicity in both cell types, with a greater potency in the colonic cancer cells. Apoptotic cell death was found to be involved in the cytotoxic effect of the most active, compound 5. Compounds 2 and 5 were found to cross the Caco-2 monolayer with apparent permeabilities above 10 x 10(-6) cm s(-1). CONCLUSIONS AND IMPLICATIONS: Compounds isolated from n-hexane extracts of E. pallida roots have a direct cytotoxicity on cancer cells and good potential for absorption in humans when taken orally