Heurtley magnifier, made by Muirhead and Company Limited, Beckenham, London, England, 1919. Ex Submarine Cable Exhibition, 1950.
This instrument was invented by E S Heurtley in 1909, and was widely used until the early 1940s, particularly by the Western Union Telegraph Company and the Commercial Cable Company, to magnify feeble electrical currents received over long-distance submarine cables. The electrical currents moves very thin wires (known as 'Wollaston wire') in and out of a hot air current, causing changes in the resistance of the wires. This unbalances the mechanism inside the magnifier, and creates higher currents which could then be used to relay the signal. It became obsolete when working valve amplifiers were introduced.