Loading coil, unknown maker, probably British, 1910-1930.

The performance of a long-distance telegraph or telephone cable can be improved by 'loading' it with additional inductance to balance the capacitance effect caused by the cable being surrounded by air (a non-conductor). The principle was conceived by Oliver Heaviside in Britain and brought into practical use by Michael Pupin in the USA. The advent of electronic repeaters eventually made loading coils unnecessary.