Shaft A of the tachometer is driven by the shaft whose speed is to be measured. Mounted on shaft A is disk 1 with holes. Disk 1 is located in a water-tight vessel 2 filled with mercury. As disk 1 rotates, the mercury begins to rotate as well and, owing to friction, tends to rotate light disk 3 which is prevented from rotating by hair spring 4. The angle through which disk 3 turns, and it is rigidly linked to hand 5, depends upon the angular velocity of the shaft being tested.
$3716$SHP,M$