In this period came the multiple switchboard, copper wire, girl operators, underground cables, metallic circuit
, common battery, and the long-distance lines.
Essentially, three types of telephone lines are used to transmit burglar alarm signals: metallic circuits, voice-grade lines, and a switched network.
Metallic circuits are pairs of copper wires that connect the protected premises directly to the central station.
Voice-grade lines are similar to metallic circuits in that they are leased from the telephone company and dedicated to alarm signal transmission.