d'Arsonval electric chronometer, for measuring reflex reaction time, by Charles Verdin, Paris, 1902

This chronometer has been adapted to record the response time of a nerve to a stimulus. The patient is touched with the ball-topped metal rod, which has been charged by an electromagnet. Once they have felt the current, they press down on the tweezers-like rod in their hand. This cuts the current and holds the needle on the dial in place so a reading in hundredths of a second can be taken.

This chronometer was invented by Jacques-Arsène d'Arsonval (1851-1940), a biophysicist. His chronometer could determine the extent and location of damage to the nervous system, as injured nerves took longer to transmit stimuli.