Rotation of high-speed spindle 1 is transmitted through gears 3 and 4 to low-speed spindle 2 and further, through bevel gears 5 and 6, to the impeller with vanes a. The impeller is keyed to the shaft of gear 6 and rotates counterclockwise about axis A. Rotating in the housing, vanes a deliver a stream of air directed against vane d which is rigidly attached to hand b. The torque on vane d is counterbalanced by spiral spring 7 which is wound up by this torque a proportional amount. Since the impeller must always rotate in the same direction, regardless of the direction of rotation of the shaft whose speed is being measured, the drive can be reversed by bringing the other gear 5 into mesh with gear 6. Gears 5 are shifted in and out of mesh by lever-hand 8 which turns about fixed axis B.
$4018$THP,M$