structure 15 for example by mounting bolts the brake coil 44 is likewise a cylindrical coil but its larger dimension is its axial dimension the electrical leads for the clutch and brake coils are indicated at 45 and 46 the magnet housing 43 bears against the end wall 47 of the bearing structure 15 via ribs 48 which leave between the magnet housing and the bearing structure a plurality of radial cooling air channels 49 additional cooling air channels 50 and 51 are located radially inward and radially outward of the magnet housing 43 respectively these channels communicate with a group of cooling air openings 52 53 in the end wall 47 a further cooling air channel 54 is located between the magnet housing 41 and the end face of the brake disk 24 which faces away from magnet housing 41 cooling air channel 54 communicates both with the cooling air channels 50 51 and also with the through bores 18 which in turn serve to establish communication with the air slots 7 the part of the structure thus far described operates as follows in the starting condition the friction layers 33 37 and 38 are axially spaced by small distances from the respective ones of the ring 5 of the flywheel 4 and the stationary.