Professor Branly found, in 1891, that if a pair of slightly-oxidised copper wires rest across one another the contact-resistance may fall from 8,000 to 7 ohms by the impact of an electric wave.
"Hertzian Wave Wireless Telegraphy"
John Ambrose Fleming
He has recently devised a tripod arrangement, in which a light metal stool with three slightly-oxidised legs stands on a polished plate of steel.
"Hertzian Wave Wireless Telegraphy"
John Ambrose Fleming
The contact points must be oxidised, but not too heavily, and the stool makes a bad electrical contact until a wave falls upon it.
"Hertzian Wave Wireless Telegraphy"
John Ambrose Fleming