Early GPO Double Current telegraph key
This is an elaborate key with a complex mechanism enclosed
within a circle. Sent out a weak negative signal between dots and dashes
to depolarize very long telegraph lines and counteract the capacitance. When not sending, the operator flipped the switch to "Receive"
allowing messages to be received over the same pair. There are several manufactors known, e.g. BI & HC Ltd, Liverpool or the G.P.O.
made keys themselves in a works not far from the Silvertown Telegraph Works in London.