Many complex machines and mechanical systems have an ideal and abstract form which has been evolved from fundamental theory without reference to those engineering principles that are so essential in practice. Thus the heat engine has its Carnot cycle and the propeller its actuator disc. With these, and numerous other examples, the object is to provide a standard of reference, to give an ideal which can be approached but never reached and, most important, to indicate the fundamental limitations imposed by theory and prevent profitless investigations.

When J. A. Brinell, at the end of the last century, interested himself in the property hardness of metals, he could have had no idea of the importance to be attained later by the new line of investigation he was launching nor of the interesting machines and their applications for which his work was to provide the inspiration.

In the aircraft industry this form of test is not solely used to determine the property hardness as such, but is used to give some idea of the maximum tensile strength of the material. This use is illustrated by some of the specifications controlling the quality of aircraft material, in which hardness tests are required on a specified percentage of bars to ensure that the bars which are riot tensile tested will give a maximum tensile strength of not less than the minimum nor greater than the maximum specification values.