Electrical conductivity in granular media and Branly's coherer: A simple experiment

accepted for publication in American Journal of Physics (to be published between February 2005 and June 2005)We show how a simple laboratory experiment can illustrate certain electrical transport properties of metallic granular media. At a low critical imposed voltage, a transition from an insulating to a conductive state is observed. This transition comes from an electro-thermal coupling in the vicinity of the microcontacts between grains where microwelding occurs. Our apparatus allows us to obtain an implicit determination of the microcontact temperature, which is analogous to the use of a resistive thermometer. The experiment also illustrates an old problem, the explanation of Branly's coherer effect - a radio wave detector used for the first wireless radio transmission, and based on the sensitivity of the metal fillings conductivity to an electromagnetic wave

Paper Session I-A - Development of a Microscopy Platform for the International Space Station

Microscopy in Zero-Gravity presents numerous challenges from slide and sample preparations to data handling and recording. A team of several Florida schools has established a distributed engineering and science team in conjunction with FSI and FSRI at Kennedy Space Center to address critical issues for an ISS microscopy payload. Flights aboard the KC-135 reduced gravity aircraft have provided engineers, faculty and students with valuable experience applied to the design of sample preparation procedures. Current plans include the development of flight hardware for flight aboard ISS in 2006. The payload includes both a high resolution scanning probe microscopy (SPM) unit and a low resolution light microscopy unit. The development of a Zero-Gravity Microscopy Analysis Platform (ZG-MAP) combines industry effort, in conjunction with academia, to deliver ISS access to both seasoned researchers and students

Paper Session I-A - Development and Testing of Scanning Probe Microscopy for the Zero-G Environment and ISS

The Association of Small Payload Researchers (ASPR) has established a partnership and collaborative effort to develop a Scanning Probe Microscope for ISS. The initiative seeks to deliver a Facility Instrument available to the entire ISS community. ASPR brings university and independent researchers together to a payload engineering forum that unites the investigator\u27s academic disciplines. The association has begun testing an atomic force microscope (AFM) aboard the KC-135 reduced gravity aircraft. The initial work included a collaboration with educational organizations that has expanded to include partnerships with space businesses and international small payload developers. The initial trials of a prototype AFM unit in zero gravity proved successful

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

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

Clinical Forms of Chikungunya in Gabon, 2010

Chikungunya fever (CHIK) is a disease caused by a virus transmitted to humans by infected mosquitos. The virus is responsible for multiple outbreaks in tropical and temperate areas worldwide, and is now a global concern. Clinical and biological features of the disease are poorly described, especially in Africa, where the disease is neglected because it is considered benign. During a recent CHIK outbreak that occurred in southeast Gabon, we prospectively studied clinical and biological features of 270 virologically confirmed cases. Fever and arthralgias were the predominant symptoms. Furthermore, variable and distinct clinical pictures including pure febrile, pure arthralgic and unusual forms (neither fever nor arthralgias) were detected. No severe forms or deaths were reported. These findings suggest that, during CHIK epidemics, some patients may not have classical symptoms (fever and arthralgias). Local surveillance is needed to detect any changes in the pathogenicity of this virus

Calcul de la largeur des franges dans l'expérience des deux miroirs

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Sur la formule des réseaux plans

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Incidencia de la escabiosis en la población escolar de la ciudad de Cuenca

Doctor en Medicina y CirugíaCuenc