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.
More info: http://www.morsemad.com/dc.htm