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.
4-12), who estimates the accuracy attainable with the duplex micrometer as equal to that of the heliometer; but as few measures of permanent value have been made with the instrument, and those made exhibit an accuracy far inferior to that of the heliometer, it is unnecessary to describe the instrument here in greater detail.
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.