Telephone users could hear the clicks of telegraph traffic going on in neighbouring wires. Experienced telegraphists, listening on the telephone, could read out the messages they heard in this way. On telephone lines passing through extremely busy groups of telegraph wires, the telegraph traffic could be heard as a continuous static, like the sound of frying. Later, it was noticed that telephone lines in close proximity similarly affected each other so that the speakers on one line could hear the conversation on the other line. The name given to this phenomenon was induction.
Induction arises because of the linked nature of electricity and magnetism.
This is the principle of the electro-magnet, which by coiling the electrical wire, directs and increases the magnetic effect, which only operates when electricity is passing through the coil. Later, it was found that a changing magnetic field similarly produces an electrical current in a piece of wire within the field. In the phenomenon of induction, both of these effects are operating. Take two wires, laid parallel to each other. Crosstalk on telephone wires was found worst where telephone lines ran over long distances on the same poles.