Laboratoire de Physique, École Normale Supérieure de Lyon, 46 allée d'Italie, 69364 Lyon cedex 07, France

Electrical phenomena in granular materials related to an electrical conduction transition (known as the Branly effect) have been interpreted in many different ways but without a clear demonstration. We report the observation of electrical transport through a chain of oxidized metallic beads. A transition from an insulating to a conducting state is observed as the applied current is increased.  We show that this transition comes from an electro-thermal coupling in the vicinity of the microcontacts between beads where microwelding occurs. An implicit determination of  the microcontact temperature is obtained. In a metallic powder, temporal fluctuations of resistance are additionally observed, and are studied with the tools used in hydrodynamic turbulence. We show that this electric noise displays scale invariance (over 4 decades in time) and intermittency, which trace back to thermal expansion locally creating or destroying electrical contacts. As in the case of hydrodynamic turbulence, the noise properties are well described by a multiplicative cascade model.