A chronometric tachometer, which measures the rpm of a shaft during three seconds, incorporates an element of clockwork. When a button (not shown) is pressed, ratchet wheel 1 begins to rotate. Its speed is controlled by balance wheel 2 and double-ended pawl 3. Cam 4, keyed to the shaft of wheel 1, turns through a certain angle and then releases latch 5 for exactly three seconds. As a result, ratchet wheel 6 and hand 7, connected by shaft A to the shaft whose speed is to be measured, turn for three seconds through the corresponding angle. The tachometer scale is graduated so that hand 7 indicates the number of shaft revolutions per minute rather than per three seconds.
$2753$RG,M$