Input shaft A is driven by the shaft being tested. By means of worm 1, worm wheel 2 and unidirectional device 3, rotation is transmitted to gear 4 always in the same direction. Gear 4 is connected through a frictional drive to ratchet wheel 5 which carries hand 6 mounted on its shaft. The mechanism is designed so that the hand always deviates in the same direction, regardless of the direction of rotation of the shaft being tested. If worm wheel 2 rotates clockwise, quadrant 7 turns clockwise and gear 7' is brought into mesh with gear 4, rotating it clockwise. If worm wheel 2 rotates counterclockwise, quadrant 7 is turned counterclockwise and gear 7" is brought into mesh with gear 8 which, in turn, meshes with gear 4, rotating it clockwise again (gears 7' and 7" rotate freely on studs of quadrant 7). Hand 6 indicates the angular velocity of the shaft being tested during a definite time interval set up by the clockwork. When button 9 is pressed, gear segment 10 is turned, winding spring 11. Gear segment 10 rotates gear 12 which is rigidly attacned to ratchet-tooth collar 13 and turns freely on shaft B. When button 9 is released, spring 11 turns gear segment 10, gear 12 and ratchet collar 13 in the opposite direction. Then a tooth of ratchet collar 13 is engaged by a pawl on escape wheel 15 which is keyed to shaft B. The pallets of anchor 14 engage the teeth of escape wheel 15 so that shaft B has a constant angular displacement perunit time. Cam 16 is keyed to shaft B and, as the shaft rotates, it retracts pawl 17 from ratchet wheel 5. Wheel 5 begins to rotate at an angular velocity proportional to the speed of the shaft being tested. After a definite time interval, cam 16 releases pawl 17 which locks wheel 5. Hand 6 is zeroed by pressing button 18.
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