In this period came the multiple switchboard, copper wire, girl operators, underground cables, metallic circuit
, common battery, and the long-distance lines.
If the conductivities of the conducting strip and ground plane surfaces are known, the metallic circuit
losses can be calculated as developed in Appendix B.
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.