Indeed, as a matter of fact, it was found that by winding the polar relay differentially and arranging the circuits and collateral appliances appropriately, the polar duplex system was more highly efficient than the neutral system, and it is extensively used to the present day.
"Edison, His Life and Inventions"
Frank Lewis Dyer and Thomas Commerford Martin
It will be understood, of course, that the polar relay, as used in the quadruplex system, is wound differentially, and therefore its operation is somewhat similar in principle to that of the differentially wound neutral relay, in that it does not respond to the operation of the key at the home office, but only operates in response to the movements of the distant key.
"Edison, His Life and Inventions"
Frank Lewis Dyer and Thomas Commerford Martin
Without specifically describing here the entire mechanism-since this is not a text-book or a treatise-we may say that a duplex telegraph-line is fitted with these differentially wound electromagnets at every station.
"Electricity and Magnetism Nature's Miracles, Vol. III."
Elisha Gray