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

TL;DR

This paper presents a simple experiment demonstrating electrical transport in metallic granular media, highlighting a transition from insulating to conductive states due to electro-thermal effects at microcontacts, and explaining Branly's coherer effect.

Contribution

It introduces an accessible laboratory setup to study electrical conduction and thermal effects in granular media, linking historical radio detection to modern understanding.

Findings

Transition from insulating to conductive state at low voltage

Electro-thermal coupling causes microwelding at microcontacts

Explains Branly's coherer effect as a sensitivity to electromagnetic waves

Abstract

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.