Ball 4 closes the orifice of bushing 3 to which compressed air is delivered. Upon deformation of the test specimen, on which the legs of the tensometer (strain gauge) rest, movable lever 2, which can turn about point A of body 1, forces back ball 4 so that air is released from bushing 3 through the annular clearance. The pressure inside bushing 3, which is measured, depends upon the displacement of movable lever 2.
$3926$LHP,M$