1. H. Hertz: Miscellaneous Papers. English translation by D.E. Jones, G.E. Schott (MacMillan, New York, 1896) pp. 163-183
2. F. Auerbach: Smithsonian Report for 1891 (Government Printing Office, Washington 1893) pp. 207–236
3. J.A. Brinnnel: II.Cong. Int. Methodes dʼEssai, J. Iron Steel Inst. 59, 243 (1901), translated to English by A. Wahlberg
4. E. Meyer: Untersuchungen über Härteprüfung und Härte, Z. Ver. Dtsch. Ing. 52, 645–654 (1908)
5. S.L. Hoyt: The ball indentation hardness test, Trans. Am. Soc. Steel Treating, 6, 396 (1924)