  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.
