Member 3 ends in sharp point a. Rocking prism 2 ends in sharp point b. The distance l between points a and b is the length being measured on piece 1 being tested. Upon a change in length l, prism 2, turning on its knife edge with respect to member 3 moves hand 4, secured in the centre of prism 2, thereby indicating the amount of strain on scale 5.
$476$SL,M$