Typically they are steam pumps, the steam and water cylinders being set tandem on the same bed frame, generally without fly-wheel or other rotary parts; they may be single cylinder or duplex, simple, compound or triple expansion, and having a higher speed of stroke are smaller in all their parts than Cornish pumps.
Grubb's duplex micrometer is described in the 9th edition of the Encyclopaedia Britannica.
The coils of the electromagnets are differentially wound with silk-covered wire, 4 mils (= 004 inch) in diameter, to a total resistance of 400 ohms. This differential winding enables the instrument to be used for " duplex " working, but the connexions of the wires to the terminal screws are such that the relay can be used for ordinary single working.
Duplex telegraphy consists in the simultaneous transmission of two messages, one in each direction, over the same wire.