A key designed by K.F. Braun  of Germany for operation with spark transmitters. An electromagnet inside the base is energized until the tension across the contacts is reduced to a minimum (an iron piece attached to the arm completes the magnetic circuit) this liberates the arm and avoids arcing. Many thanks to Ing. C. Lualdi for this donation and to Dipl. Ing. Gregor Ulsamer who supplied the explanation of how this key works.