In 1737, his festival (duplex) being observed on the 19th of July.
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.
The first to introduce a really good practical system of duplex telegraphy, in which this difficulty was sufficiently overcome for land line purposes, was J.