Editing Operational definition (section)

=== Mechanical hardness ===
Unlike [[temperature]] and [[electric current]], there is no abstract physical concept of the [[hardness]] of a material. It is a slightly vague, subjective idea, somewhat like the idea of [[intelligence (trait)|intelligence]]. In fact, it leads to three more specific ideas:

# Scratch hardness measured on [[Mohs scale of mineral hardness|Mohs' scale]];
# Indentation hardness; and
# Rebound, or dynamic, hardness measured with a Shore [[scleroscope]].

Of these, indentation hardness itself leads to many operational definitions, the most important of which are:

# [[Brinell hardness test]] – using a 10&nbsp;mm steel ball;
# [[Vickers hardness test]] – using a pyramidal diamond indenter; and
# [[Rockwell hardness test]] – using a diamond cone indenter.

In all these, a process is defined for loading the indenter, measuring the resulting indentation, and calculating a hardness number. Each of these three sequences of measurement operations produces numbers that are consistent with our subjective idea of hardness. The harder the material to our informal perception, the greater the number it will achieve on our respective hardness scales. Furthermore, experimental results obtained using these measurement methods has shown that the hardness number can be used to predict the stress required to permanently deform steel, a characteristic that fits in well with our idea of resistance to permanent deformation. However, there is not always a simple relationship between the various hardness scales. Vickers and Rockwell hardness numbers exhibit qualitatively different behaviour when used to describe some materials and phenomena.