The circuit is made and broken by the revolution of the disc causing the pins to enter and leave the mercury.
"Hertzian Wave Wireless Telegraphy"
John Ambrose Fleming
He could mark them signalling to each other, then as two or three of them rose to peer forward, he saw on their foreheads and chests the blood-red disc, the ominous distinguishing mark of the Igazipuza.
"The Luck of Gerard Ridgeley"
Bertram Mitford
He glanced at the luminous disc of his watch.
"The Orchard of Tears"
Sax Rohmer