Both the telegraph system and the telephone system used a technology called circuit switching to create the paths through which electricity travels along the wire.

As a basic principle, electricity will flow from point A to point B only as long as the two points have a continuous connection. If that connection breaks, electricity stops flowing, and the signal is lost. The connection between the two points is called a circuit; when the circuit is closed, electricity flows across the connection. When the circuit is broken or disrupted, the signal stops.