Some aspects of electrical conduction in granular systems of various dimensions

We report on measurements of the electrical conductivity in both a 2D triangular lattice of metallic beads and in a chain of beads. The voltage/current characteristics are qualitatively similar in both experiments. At low applied current, the voltage is found to increase logarithmically in a good agreement with a model of widely distributed resistances in series. At high enough current, the voltage saturates due to the local welding of microcontacts between beads. The frequency dependence of the saturation voltage gives an estimate of the size of these welded microcontacts. The DC value of the saturation voltage (~ 0.4 V per contact) gives an indirect measure of the number of welded contact carrying the current within the 2D lattice. Also, a new measurement technique provides a map of the current paths within the 2D lattice of beads. For an isotropic compression of the 2D granular medium, the current paths are localized in few discrete linear paths. This quasi-onedimensional nature of the electrical conductivity thus explains the similarity between the characteristics in the 1D and 2D systems.Comment: To be published in The European Physical Journal

Effects of electromagnetic waves on the electrical properties of contacts between grains

A DC electrical current is injected through a chain of metallic beads. The electrical resistances of each bead-bead contacts are measured. At low current, the distribution of these resistances is large and log-normal. At high enough current, the resistance distribution becomes sharp and Gaussian due to the creation of microweldings between some beads. The action of nearby electromagnetic waves (sparks) on the electrical conductivity of the chain is also studied. The spark effect is to lower the resistance values of the more resistive contacts, the best conductive ones remaining unaffected by the spark production. The spark is able to induce through the chain a current enough to create microweldings between some beads. This explains why the electrical resistance of a granular medium is so sensitive to the electromagnetic waves produced in its vicinity.Comment: 4 pages, 5 figure

Palyno-stratigraphy of the lower Karroo sequence in the central Sebungwe District, Mid-Zambezi Basin, Rhodesia

Hart (1962) was the first to discuss seriously the subject "palynology-key to stratigraphy" with regard to Southern Africa. Since then a number of publications have appeared on African Karroo palynology. However, to date only one large scale microfloral zonation scheme has been proposed -that of Hart (1967). In this, four major palynofloristic zones were outlined for Lower Karroo (Permian) sequences, drawn from surface and sub-surface material in South and Central Africa. Another scheme, dividing the Permian into eight zones in S. Africa (Great Karroo Basin) , is as yet unpublished (Anderson, in press). In an attempt to apply palynology to the problems of geological correlation and relative age determinations, specifically in the field of coal exploration in Rhodesia, a standard section in the form of one borehole, the Matabola Flats borehole, well-sited, deep and with apparently continuous deposition was chosen for palynological biostratigraphic analysis. Fifty-five miospore genera and ninety-eight species have been recognised in this study. Their systematic descriptions and statistical analyses are published elsewhere, Falcon 1975 a, b. Detailed analysis of the forty-eight productive samples show large scale micro-floral changes up the stratigraphic column. Four major assemblage zones and eight assemblage sub-zones are herein proposed, thereby expanding by four the palynofloristic zones of Hart.CSI

Discussion on: numerical methods in the definition of palynological assemblage zones in the Lower Karroo (Gondwana) of Rhodesia

Main articleSubsequent to the publication of the above paper (in which a number of errata appeared) certain points of discussion have been raised regarding the interpretation of the single axis spatial ordinations. In particular delineation of the numerical assemblage zones based upon the spacings between the clustered groups has been in question. In order to understand fully the significance of these groupings the following points should be borne in mind.Non

Aspects of palynology in Rhodesia

Main articleThis paper is based on a Doctoral Thesis presented to the University of the Witwatersrand, which in whole or part will be published elsewhere at greater length. The essence of the research is presented in three text figures which show the proposed correlations of Karoo strata on opposite sides of the Rhodesian palaeowatershed, correlations with Karoo-equivalent strata in other parts of Gondwanaland, and palaeofloristic trends in Rhodesia during the Permo-Triassic.Non

Effects of pyruvate administration on infarct volume and neurological deficits following permanent focal cerebral ischemia in rats

Recent experimental evidences indicate that pyruvate, the final metabolite of glycolysis, has a remarkable protective effect against different types of brain injury. The purpose of this study was to assess the neuroprotective effect and the neurological outcome after pyruvate administration in a model of ischemic stroke induced by permanent middle cerebral artery occlusion (pMCAO) in rats. Three doses of pyruvate (250, 500 and 1000 mg/kg, i.p.) or vehicle were administered intraperitoneally 30 min after pMCAO. In other set of experiments, pyruvate was given either before, immediately after ischemia or in a long-term administration paradigm. Functional outcome, mortality and infarct volume were determined 24 h after stroke. Even when the lowest doses of pyruvate reduced mortality and neurological deficits, no concomitant reduction in infarct volume was observed. The highest dose of pyruvate increased cortical infarction by 27% when administered 30 min after pMCAO. In addition, when pyruvate was given before pMCAO, a significant increase in neurological deficits was noticed. Surprisingly, on the contrary of what was found in the case of transient global ischemia, present findings do not support a great neuroprotective role for pyruvate in permanent focal cerebral ischemia, suggesting two distinct mechanisms involved in the effects of this glycolytic metabolite in the ischemic brain

Ripples in Tapped or Blown Powder

We observe ripples forming on the surface of a granular powder in a container submitted from below to a series of brief and distinct shocks. After a few taps, the pattern turns out to be stable against any further shock of the same amplitude. We find experimentally that the characteristic wavelength of the pattern is proportional to the amplitude of the shocks. Starting from consideration involving Darcy's law for air flow through the porous granulate and avalanche properties, we build up a semi-quantitative model which fits satisfactorily the set of experimental observations as well as a couple of additional experiments.Comment: 7 pages, four postscript figures, submitted PRL 11/19/9

Single Stellar Populations in the Near-Infrared - I. Preparation of the IRTF spectral stellar library

We present a detailed study of the stars of the IRTF spectral library to understand its full extent and reliability for use with Stellar Population (SP) modeling. The library consist of 210 stars, with a total of 292 spectra, covering the wavelength range of 0.94 to 2.41 micron at a resolution R = 2000. For every star we infer the effective temperature (Teff), gravity (logg) and metallicity ([Z/Zsun]) using a full-spectrum fitting approach in a section of the K band (2.19 to 2.34 micron) and temperature-NIR colour relations. We test the flux calibration of these stars by calculating their integrated colours and comparing them with the Pickles library colour-temperature relations. We also investigate the NIR colours as a function of the calculated effective temperature and compared them in colour-colour diagrams with the Pickles library. This latter test shows a good broad-band flux calibration, important for the SP models. Finally, we measure the resolution R as a function of wavelength. We find that the resolution increases as a function of lambda from about 6 angstrom in J to 10 angstrom in the red part of the K-band. With these tests we establish that the IRTF library, the largest currently available general library of stars at intermediate resolution in the NIR, is an excellent candidate to be used in stellar population models. We present these models in the next paper of this series.Comment: 17 pages, 19 figures. Accepted for publication in Astronomy and Astrophysic