This telephone enclosed its diaphragm, magnet, and coils in a
polished hard rubber case. Its design as a Magneto allowed it
to be used as a receiver and as a transmitter. The more
expensive models included two phones, one to be used as the
transmitter and the other as the receiver. The small knob on
the phone was used to activate a switch automatically cutting
the instrument out of the circuit when the phone was placed in
its holder. This feature was used in the enclosure illustrated in
the bottom left photo.
The above photo courtesy of the Smithsonian Institution
The Duplex phone on the left was used with one of Phelps' Magneto Call Boxes. By turning the crank, located on the side of the cabinet, a bell would ring at the receiving end. It was also available with a second Duplex Phone which was stored inside the enclosure. This second phone would allow for separate receiver and transmitter use. It also allowed a user to use both phones as a receiver. The user, by holding a phone in each hand, could place one to each ear. This helped out in hearing in noisy factory environments. The phone on the right included two duplex telephones as standard and were designated as separate receiver and transmitters.
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George M. Phelps