P. Béqu<strong>in</strong>, V. Tournatroughly equal or larger than the length <strong>of</strong> the cha<strong>in</strong> <strong>of</strong> beads:5 × 11 mm) shows that the current <strong>in</strong>duced by the spark isproportional to 1/r 2.1 (obta<strong>in</strong>ed through a l<strong>in</strong>ear regressionon log[I sp ]). These results are consistent with those obta<strong>in</strong>ed<strong>in</strong> [21] wherean<strong>in</strong>ducedcurrentproportionalto1/r m withm ≃ 1.2 fordistancesr vary<strong>in</strong>g between 0.1 <strong>and</strong> 2.2 mis observed. Note that the spatial dependence <strong>of</strong> monochromaticelectromagnetic wave amplitude is respectively proportionalto 1/r 2 <strong>in</strong> the near field <strong>and</strong> proportional to 1/r <strong>in</strong>the far field.At this level, the lack <strong>of</strong> <strong>in</strong>formation on the electromagneticfield (amplitude, frequency spectrum <strong>and</strong> directivity)does not allow the development <strong>of</strong> a realistic model to becompared to experimental results. These particular electromagneticbehaviors as a function <strong>of</strong> distance (near field/farfield) will have to be taken <strong>in</strong>to account <strong>in</strong> the models <strong>and</strong>experiments dedicated to the electromechanical behavior <strong>of</strong>agranularmediumwithlargedimensions.6ConclusionCont<strong>in</strong>ued <strong>in</strong>terest <strong>in</strong> granular materials has stimulated newexperiments <strong>and</strong> models to characterize these complex structures.In this article the electrical <strong>and</strong> thermal behaviors <strong>of</strong> acha<strong>in</strong> <strong>of</strong> beads carry<strong>in</strong>g a dc <strong>and</strong> an impulse electric currenthave been <strong>in</strong>vestigated. This work provides theoretical <strong>and</strong>experimental cont<strong>in</strong>uations to the studies <strong>of</strong> Refs. [13,14,21]mostly carried out <strong>in</strong> the context <strong>of</strong> the Branly <strong>effect</strong>.The passage <strong>of</strong> an electric current <strong>in</strong> a cha<strong>in</strong> <strong>of</strong> beadsaffects the nature <strong>of</strong> the contact between beads by caus<strong>in</strong>gaheat<strong>in</strong>g<strong>and</strong>as<strong>of</strong>ten<strong>in</strong>g<strong>of</strong>themetalnearthe<strong>in</strong>terface.These changes are consequent to the heat which is generated<strong>in</strong> the constricted region <strong>of</strong> the current flow <strong>and</strong> lead to an<strong>in</strong>crease <strong>in</strong> the contact surface. We developed specifically anexperimental setup suited to the measurement <strong>of</strong> the electriccurrent <strong>and</strong> voltage for a cha<strong>in</strong> <strong>of</strong> metallic beads. The analysis<strong>of</strong> the voltage–current characteristics <strong>of</strong> such a cha<strong>in</strong><strong>in</strong>dicates that the diameter <strong>of</strong> the contact surface varies l<strong>in</strong>earlywith electric current I .Subjectedtoacycliccurrent,the cha<strong>in</strong> <strong>of</strong> beads presents an electromechanical behavior<strong>of</strong> hysteretic type. The larger are the electric current <strong>and</strong> thenumber <strong>of</strong> beads the larger is the hysteretic <strong>effect</strong> (the loop)<strong>and</strong> the greater is the heat released <strong>in</strong> the cha<strong>in</strong>.Steady states models derived on the assumption <strong>of</strong> equilibriumbetween <strong>Joule</strong> heat<strong>in</strong>g <strong>in</strong> the contact area <strong>and</strong>thermal dissipation by <strong>conduction</strong> with<strong>in</strong> the material areproposed. The simplified models <strong>of</strong> Eq. 8 <strong>and</strong> the modeldeveloped <strong>in</strong> [13,14], all based on the Wiedemann–Franz’slaw (W–F) (which states that the product <strong>of</strong> the thermal conductivity<strong>and</strong> the electrical resistivity <strong>of</strong> a metal is proportionalto the temperature), are observed to be <strong>in</strong>adequate todescribe electromechanical <strong>conduction</strong> <strong>in</strong> cha<strong>in</strong>s <strong>of</strong> beadswith cyclic currents under a weak static compression force∼1N.AnextendedmodelEq.6 is then developed without therestrictions <strong>of</strong> the W–F’s law. The comparisons with experimentaldata provide the variations <strong>of</strong> parameters α <strong>and</strong> βcorrespond<strong>in</strong>g respectively to the l<strong>in</strong>ear evolution <strong>of</strong> the electricalresistivity <strong>and</strong> to the thermal conductivity (Eq. 5). Theseresults could benefit <strong>in</strong> the future from temperature measurementson the beads <strong>and</strong> from the use <strong>of</strong> various materials <strong>and</strong>bead diameters.The <strong>effect</strong> <strong>of</strong> an electromagnetic pulse produc<strong>in</strong>g a pulse<strong>of</strong> electric current through a cha<strong>in</strong> <strong>of</strong> beads is f<strong>in</strong>ally studied.The electromagnetic wave pulse is created by an electric discharge(electric spark) placed near the cha<strong>in</strong> <strong>of</strong> beads. Theexperimental results show that the current can cause appreciablechanges <strong>in</strong> the contact surface when the magnitude <strong>of</strong>the <strong>in</strong>duced current is sufficient but also when the dccurrent rema<strong>in</strong> weak. Evolution <strong>of</strong> the <strong>in</strong>duced current withthe cha<strong>in</strong>/spark distance <strong>in</strong> near field (100 mm) <strong>in</strong> Ref.[21] wherethe1/r dependence is observed.The control <strong>of</strong> the contact surface between beads us<strong>in</strong>gasuitableelectromagneticwavecouldf<strong>in</strong>dapplications<strong>in</strong>particular <strong>in</strong> the study <strong>of</strong> the fundamental processes <strong>in</strong>volved<strong>in</strong> the acoustic propagation through granular media [29–32].Acknowledgments We would like to thank James Blondeau, StéphaneLebon <strong>and</strong> Eric Egon for their assistance <strong>in</strong> the experiments <strong>and</strong>their technical advice. We particularly thank Jean-Marie Genet for hisexperimental work.References1. Duran, J.: S<strong>and</strong>s, Powders <strong>and</strong> Gra<strong>in</strong>s. Spr<strong>in</strong>ger-Verlag, NewYork (1999)2. 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