A Kurbelinduktor is a hand-operated dynamo, similar to a crank.  By turning it, an electrical current is generated.

The first telephones required the user to turn the handle to create the current that opened the flap on the Klappenschrank.

These days the Kurbelinduktor technology can be found in wind-up mobile phone chargers and torches.

To hear a simple explanation and a short discussion in German, listen to the podcast:

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Download the MP3 file