The Lodge-Muirhead coherer was patented in 1902, and the present example is thought to be a prototype of that time. When operational, the mercury has a covering layer of oil which is just in contact with the revolving wheel. An incoming radio pulse sets up electrostatic forces between the wheel and the mercury which squeeze out the oil and give low-resistance contact.
The Poldhu transmitter is famous as the site of the first transatlantic radio signal (the letter S) in December, 1901.