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In 1912 Henri Piéron (1881–1964) became Chairman of the Physiological Psychology Laboratory at the Sorbonne following the death of Alfred Binet (1857–1911). At that time, he decided to refocus his experimental research on the field of psychochronometry, with the use of two of the most famous tools for the measuring of reaction times (RT): The Hipp chronoscope and the d’Arsonval chronometer. Although German and American psychologists favored the use of the chronoscope invented by Matthäus Hipp (1813–1893), French psychologists preferred to use the chronometer invented in 1886 by Jacques Arsène d’Arsonval (1851–1940), because of its maneuverability and portability. It was with the use of these two devices, which are presented in the first part of this paper, that Piéron studied the law put forward by Wundt in 1874, according to which RT varies inversely with intensity of stimulation, and increasingly so in the threshold region. Piéron was the first to mathematically formulate this law, now known as Piéron’s Law, and to apply it to the different senses. According to Piéron’s law, the decrease in RT is hyperbolic in nature, and reaches a plateau where it is no longer influenced by intensity, the decrease being inversely proportional to some power of the stimulus intensity. Piéron’s law is still widely cited in current work in psychophysiology.