Braun Cosmophon BMF 2020

This microphone consists of a carbon powder packed into a Bakelite capsule, which is mounted in a ring by four springs. A battery is used to apply a voltage across the carbon, and the resistance is modulated by soundwaves, converting acoustic energy into an electrical signal. It has an indicator lamp and a transformer to isolate the battery from the next stage of the signal chain and to convert the impedance.

This microphone is approaching 100 years old and amazingly still works - albeit with a lot of background noise.

Translated from the German advertisiment:

Signal lamp, built-in input transformer, and built-in, easily replaceable dry battery (standard 4.5 volt flashlight battery). On/off switch.

This microphone has the technical advantages described above and is mounted on an adjustable stand with a molded-material base.

An extremely practical design for dictation microphones, command center microphones, and use with home sound recorders.

The built-in signal lamp lights up automatically when the microphone power is switched on. Each microphone comes with a basic circuit diagram, as well as detailed operating instructions with all the information needed to achieve flawless transmission of music and speech.

The Microphone is in a high-gloss chrome-plated frame, suspended without shaking, with connection sockets and 6 mm tripod terminal. A particularly high-quality contact microphone with non-backed carbonides, therefore flawless, noise-free and constantly loud reproduction of music and speech. Operating voltage 4 volts, approx. 200 MA. Especially suitable for recording, music and voice transmissions, amateur radio. Loud playback, even without preamplifier.

A post about the Cosmophone at Vintage Microphone World