Blattnerfon is an apparatus used in radio technique in the 30s and early 40s for magnetic recording of sound. The carrier for recording is a steel tape from magnetic tungsten steel that is highly polished on its surface and protected against ageing by thermal treatment. The tape is 3 mm wide and 0.08 mm thick. Its speed is 1.5 m/s. An aluminium reel of the diameter 62 cm holds 2700 m of the tape. The reproduction time of a full reel is about 30 minutes. The Radio used two sets of this type (one for recording, one for reproduction), both of them being in the Museum collection.