One of the first instruments to be used by psychologists was the manual dynamometer, which was first used to measure the physical strength of individuals before later coming to be a tool for measuring “mental strength”. In fact, all of the first mental tests to be invented (e.g. Cattell, 1890) consisted in psycho-physical tests of manual strength using a dynamometer. Here we shall present a history of the technical development of the dynamometer, while also taking account of its interest for psychology. By far the best known dynamometer at the turn of the 20th century was the instrument made and sold by the mechanical engineer Anatole Collin. The size, shape and method of this dynamometer were the result of a whole series of improvements introduced since the instrument invented by Regnier (1798) and then transformed, in particular during the 19th century, by the French physicians Burq and Duchenne de Boulogne. As of the middle of the 19th century, the catalogs of many makers, in particular in France (e.g. Lüer, Charrière, Mathieu), were to include dynamometers. However, the instrument that was to become most firmly established was the one made by Collin and used, in particular, in the experimental work of Binet and other psychologists of renown. This instrument was to become an indispensable tool not only for psychologists but also for physicians, physiologists, anthropologists and medical doctors within the context of their work.