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CHROME RING TELEPHONE CABLE, 1930's
Sample of underground telephone cable.
The wires are insulated with paper and the cable were laid in airtight ducts, air was pumped in occasionally to check for leaks, as moisture caused problems with the insulation. the outer covering is Gutta Percha.
This sample is held together by a chrome ring for display purposes.
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- Trunk cables were permanently pressurized and the pressure monitored so that an alarm would indicate to the nearest exchange that a leak had happened. The location of the leak was detected by going along to the manholes/footway boxes and measuring the local pressure with a manometer. The place with the lowest pressure indicated the nearest one to the leak. Valves the same as on car tyres were fitted for this.
I could be wrong, but I think the pressure was around 9psi. There were also flow-meters fitted at the exchange. Leaks had to be fixed quickly as drying out the cable was difficult.
.......... Bob Foudy, Cape Town RSA, 15th of December 2020