What does the C scale in hardness testing represent?

The C scale in hardness testing, specifically in the Rockwell hardness test, signifies the use of a diamond indenter for determining hardness. The diamond indenter is necessary for testing harder materials, as it can penetrate and ensure accurate readings without deformation from softer materials. This scale measures the depth of penetration under a large load compared to a minor load, providing a measure of hardness that is particularly suited for harder materials such as hardened steel.

The other options refer to different indenters that are used for various hardness testing scales but do not apply to the C scale specifically. For instance, a flat indenter is used in different hardness tests (like the Shore hardness test), a cone indenter might refer to other specific tests that do not correlate with the C scale, and a ball indenter is typically associated with the Brinell hardness test. The characteristics and applications of these indenters define their respective roles in hardness measurements, but the diamond indenter is uniquely essential for the C scale.