Ericsson 'special protection' industrial phone

Magneto wall telephone with special protection against high-tension currents, polished walnut case, interior lined with earthed iron sheets (partially rusty), 5-magnet crank inductor, insulated ebonite crank, double bell, dummy handset, the sound being carried by means of 2 rubber tubes to transmitter and receiver in the case. Ericsson marketed telephones with "special protection against high tension currents" (high voltage) primarily for power companies, railways, and industrial sites. At the time, these organizations were running telephone lines in close proximity to high-voltage power transmission lines, creating significant safety and technical risks ('induced currents' and surges).