The Single-Wire-Telephone (SWT) has replaced the Army "field telephones" as the weapon of choice for wired cave and mine rescue communications.  The devices are very simple, using a single op-amp to both send and receive an audio signal (voice) along a single insulated wire, using the operators coupling capacitance to ground for the return.  The receiver has a very high input impedance.  Power demand is very low, with many hours (or days) of operation from a 9 Volt battery.  Very little coupling to ground is required at the receive end.  The transmit end is mainly driving the distributed capacitance of the wire to ground.  Very long wires (kilometers) will require fairly good coupling to ground at the transmit end.  Obviously, two-way communications over a very long wire will require fairly good grounds at both ends such as a small peg in dirt or a bare wire in water.  Neville Mitchie's classic circuit is shown below:

Michael Lake added a high pass filter (for power line hum) and a low pass filter to reject AM Broadcast stations, and produced and sold some very nicely constructed surface-mount SWT's.  See  for details and schematic.

A SWT was published in the British CREG Journal.  A slightly modified version is at .  This more complex design includes a standby mode and a ringer.

Desirable features in a rescue SWT include an LED "ON" indicator (only bright enough to see on a cave, could just blink); provision for a ground lead; use of AA cells for a power source (which most cavers carry) or built-in lithium cells (with a foolproof OFF switch) and waterproofness.