  We report on observations of the electrical transport within a chain of
metallic beads (slightly oxidised) under an applied stress. A transition from
an insulating to a conductive state is observed as the applied current is
increased. The voltage-current (U-I) characteristics are nonlinear and
hysteretic, and saturate to a low voltage per contact (0.4 V). Our 1D
experiment allows us to understand phenomena (such as the "Branly effect")
related to this conduction transition by focusing on the nature of the contacts
instead of the structure of the granular network. We show that this transition
comes from an electro-thermal coupling in the vicinity of the microcontacts
between each bead - the current flowing through these contact points generates
their local heating which leads to an increase of their contact areas, and thus
enhances their conduction. This current-induced temperature rise (up to 1050
deg C) results in the microsoldering of the contact points (even for voltages
as low as 0.4 V). Based on this self-regulated temperature mechanism, an
analytical expression for the nonlinear U-I back trajectory is derived, and is
found to be in very good agreement with the experiments. In addition, we can
determine the microcontact temperature with no adjustable parameters. Finally,
the stress dependence of the resistance is found to be strongly non-hertzian
due to the presence of the surface films. This dependence cannot be usually
distinguished from the one due to the disorder of the granular contact network
in 2D or 3D experiments.
