The MI-12877 was a cartridge containing thin magnetic wire for monophonic recording and playback of audio, and was introduced in 1947 by RCA for use in the RCA Magnetic Wire Recorder (model MI-12875). The RCA machine was most likely intended for voice recording and a separate accessory tabletop microphone, MI-6226N, was also available.
Wire was already in use as a recording medium and was at its most popular around this time but it was fragile and needed to be threaded across the recording head and attached to a take-up spool. The MI-12877 cartridge made using wire much easier.
The recording time was 30 mins with the wire travelling at 61 cm a second – an indicator on the rear of the casing indicated the remaining length. Two portions of the wire were exposed at the front of the cartridge, with one portion being erased while the other portion is then recorded to. When the wire reached the end, the motor stopped automatically.
It’s unclear when production stopped, but wire recording was eclipsed by tape recording in the early 1950s.