Strowger calling dial, made by the Automatic Electric Company, United States, 1900.
This calling dial was the first design to use the 'three wire' system which had an earth connection in addition to the two telephone lines connecting to the exchange. Dialling impulses were sent alternately along the two exchange wires, and the earth connection provided the return path. The 'three wire' system was standard until 1907.
- Science Museum, Information Age Gallery: Exchange
- Object Number:
Mount with calling dial attached: 185 mm x 38 mm x 51 mm,
- telephone component
- component - object
- Donated by BT Heritage and Archives
- Permanent collection
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