The German brustmikrofon (chest microphone) plugs into a field telephone using the same 5-pin socket as the standard handset. Unlike the handset, there is no push-to-talk button on the brustmikrofon. The push-to-talk button on the handset conserves the telephone’s battery which powers the microphone capsule. So how is the battery conserved when the brustmikrofon is plugged in?
Well, if you remove the back cover from the brustmikrofon, there is a small switch located at the top. This is in the red circle in the photograph below:
The switch contacts are actuated by a cam lever connected to the speaker horn. The microphone capsule is removed in the photo below to show this lever:
When the speaker horn is put in the down position, the cam lever opens the switch contacts, and the microphone is disconnected from the battery.
When the speaker horn is raised all the way, the cam lever closes the switch contacts, and the microphone is connected to the battery. The microphone will pick up sound.
When the conversation is over, the speaker horn is lowered to disconnect the microphone and conserve the battery.
An electrical schematic for the brustmicrofon is shown below…
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WW2 German Field Telephone Equipment 