The rotation of point a, held firmly in contact with the centre-punched recess at the end of the shaft whose revolutions are to be counted, is transmitted by worm 1 to worm wheels 3 and 2 which have 100 and 101 teeth, respectively. The difference in the number of revolutions they make is used to determine the number of revolutions of the shaft being tested. Wheel 3 carries hand b and wheel 2, a dial scale.
$2833$WG,M$