Heaviside in 1887 succeeded in communicating by telephonic speech between the surface of the earth and the subterranean galleries of the Broomhill collieries, 350 feet deep, by laying above and below ground two complete metallic circuits, each about 24 m.
By stretching on the island and mainland parallel wire circuits earthed at each end, good telephonic communication over an average distance of 62 m.
An exchange is a central station to which wires are brought from the various subscribers in its neighbourhood, any two of whom can be put in telephonic communication with each other when the proper pairs of wires are joined together in the exchange.
Although the bells are constantly in circuit their high impedance prevents any appreciable interference with the telephonic currents.
In circuits possessing high resistance and capacity and low inductance per mile, telephonic currents are rapidly attenuated, and the higher the frequency the more rapid is the attenuation.