It is fitted with a guard plate, which works loosely until the penetration has progressed to a depth of 0.222 inch, whereupon it tightens.

(See Fig.

43.) The effect is that of sinking a ball half its diameter into the specimen.

This apparatus is fitted into the movable head of the static testing machine.
[Illustration: FIG.

43 .-- Design of tool for testing the hardness of woods by indentation.] The wood to be tested is cut square with the grain into rectangular blocks measuring 2" X 2" X 6".