STC 4017-C Microphone in factory box, Ser. No. 1166, c.1930.

This microphone was first introduced by the Standard Telephone Company and BBC in 1938. This moving coil microphone was particularly used for outside broadcasting and remained in use until the mid-1950s.

In moving coil microphones, a coil of wire surrounds a magnet, and is connected to a diaphragm which vibrates in response to sound waves. As the diaphragm moves, the attached coil vibrates back and forth around the magnet, generating an electrical current, which creates the audio signal.

The 4017-C replaced the unreliable plug and socket connection of its predecessor, the 4017-A, with more secure screw terminals.