The Heurtley Magnifier
This is an extract from from Submarine Telegraphy by Bernard S Finn, HMSO 1973, now out of print and originally published by the Science Museum in 1973.
...Much effort was spent in trying to design instruments that would relay the message automatically. To do this the signal first had to be amplified, or 'magnified'. One popular magnifier, widely used on Western Union lines, was developed by E. S. Heurtley in 1908. The moving coil, similar to that of the siphon recorder, was attached to two wires. The wires were heated by the electric current flowing through them and cooled by two small blowers. When the coil turned, one wire moved closer to its blower, the other moved away. Since resistance depends on temperature, the resistance of the first wire decreased and that of the other increased. This upset the balance of an electrical circuit of which the two wires were a part, and produced an amplified signal. The output of the Heurtley magnifier was large enough to operate a relay which produced a signal strong enough to send on to the next station. One form of relay, designed by S. G. Brown, also operated on the moving coil principle; the coil moved a needle which rested on a rotating drum (rotating in order to reduce the friction). There were two conducting bands on the drum, separated by a neutral area, and as the needle moved back and forth under the influence of the coil it would make contact first with one band, then with the other. The Heurtley magnifier and Brown drum relay came into widespread use after the first world war.