because the impact will
develop a force of several hundred thousand pounds per square inch
under the tiny diamond-pointed hammer head, stressing the test
piece at this point of contact much beyond its ultimate strength.
The rebound is thus dependent upon the indentation hardness, for
the reason that the less the indentation, the more energy will
reappear in the rebound; also, the less the indentation, the harder
the material. Consequently, the harder the material, the more the
rebound.

"Indentation hardness" is a measure of a material's resistance
to penetration and deformation. The standard testing machine is
the Brinell, Fig. 12. A hardened steel ball, 10 mm. in diameter,
is forced into the test piece with a pressure of 3,000 kg. (3-1/3
tons). The resulting indentation is then measured.

[Illustration: FIG. 12.--Hydraulic testing machine. (Brinell
principle.)]

While under load, the steel ball in a Brinell machine naturally
flattens somewhat. The indentation left behind in the test piece is
a duplicate of the surface which made it, and is usually regarded
as being the segment of a sphere of somewhat larger radius than
the ball. The radius of curvature of this spherical indentation
will vary slightly with the load and the depth of indentation.
The Brinell hardness numeral is the quotient found by dividing the
test pressure in kilograms by the spherical area of the indentation.
The denominator, as before, will vary according to the size of the
sphere, the hardness of the sphere and the load. These items have
been standardized, and the following table has been constructed
so that if the diameter of the identation produced by a load of
3,000 kg. be measured the hardness numeral is found directly.

TABLE FOR BRINELL BALL TEST
------------------------------------------------------------------------
Diameter of Ball | Hardness Number | Diameter of Ball | Hardness Number
Impression, mm. | for a Load of | Impression, mm. | for a Load of
| 3,000 kg. | | 3,000 kg.
-----------------|-----------------|------------------|-----------------
2.0 | 946 | 4.5 | 179
2.1 | 857 | 4.6 | 170
2.2 | 782 | 4 7 | 163
2.3 | 713 | 4.8 | 156
2.4 | 652 | 4.9 |