The hardness of a metal often is of prime importance. The ideas of scratching, cutting, indentation, transient impact, and permanent deformation all are conveyed in the term hardness. Various measuring instruments have been devised; of these the Brinell ball-indentation machine and the Shore scleroscope are the most common.
In using the Brinell machine which is shown in Fig. 3, the steel ball, 10 mm. in diameter, is pressed into the metal with a force of 3000 kg. The size of the indentation after applying the pressure for 15 seconds measures the hardness.
As illustrated in Fig. 4, this instrument, operated pneumatically, lets fall a weight whose diamond point cuts the surface of the metal, yet is blunt enough to be quickly resisted by the spreading metal; the upward rebound is measured on a scale whose 100 mark is exactly equaled on a hard standard steel.