To insert gauging head 1 into the bore to be measured, sensitive contact 2 is retracted by pushing button 3 which turns lever 4 clockwise about fixed axis A. End a of lever 4, runs up against a lug of lever 5, turning the latter counterclockwise about fixed axis B so that its ball tip d forces screw b to the right, retracting sleeve 6 and sensitive contact 2. When button 3 is released, springs 7 and 8 return lever 5 to its vertical position, and lever 5 and spring 9 advance sensitive contact 2 to the left. Screwed to gauging head 1 is plate e whose edge is ground spherical to a radius somewhat less than the radius of the bore to be measured. Since, the part of the plate at contact 2 is cut off, the gauge will contact the bore at two points f, located at 120° from each other, and at the spherical tip of sensitive contact 2. Deviations of the bore diameter from its nominal size are indicated on scale k.
$1776$GL,M$