Voice communications system for breathing mask - has two=wire cable and adaptor connecting mask voice unit to cage telephone set

摘要

The voice communications system esp. for miners has the unit (1) in the mask connected via an adaptor (3) to a station above ground. The adaptor is connected to the telephone set (4) belonging to the cage. The cage telephone set is coupled via the cage cable (5) to the above-ground station. A two-wire cable (2) connects the mask voice unit to the cage adaptor (3). The two wires are connected to the primary and secondary windings respectively of two transformers coupled at their secondary and primary windings to amplifiers.