Helmholtz clocked nerve signals with a frog's leg and a mirror
In 1849 most scientists assumed nerve impulses travelled too fast to measure. Hermann von Helmholtz hooked a frog's sciatic nerve to its calf muscle, bounced light off a galvanometer mirror across the room, and timed the signal at roughly 25 to 38 metres per second. Two years later his ophthalmoscope made him world famous.
Born in Potsdam on 31 August 1821, the son of a gymnasium headmaster steeped in philosophy, Helmholtz wanted to study natural science, but his father pushed him into medicine, which came with financial support. He earned a medical doctorate in 1842 and interned at Berlin's Charité hospital. His career then moved through anatomy at Berlin's Academy of Arts, physiology at Königsberg, Bonn and Heidelberg, and finally, from 1871, physics at the university in Berlin.
His first landmark, an 1847 treatise on the conservation of energy, grew out of muscle research. Helmholtz wanted to show that no energy vanishes when a muscle moves, undercutting the vitalist belief in special living forces. Building on Carnot, Clapeyron and Joule, he treated heat, light, electricity, magnetism and mechanics as expressions of one underlying quantity. Credit for energy conservation is shared with Mayer, Joule and others. In the 1850s and 1860s he and William Rankine helped popularise the idea of the universe's eventual heat death, and he derived theorems on vortices in ideal fluids.
Perception was his other passion. His 1851 ophthalmoscope let doctors see inside the living eye, and his Handbook of Physiological Optics became the standard work on depth, colour and motion perception, revised repeatedly amid a long feud with Ewald Hering that split physiology. He showed that perceived loudness does not rise in step with sound pressure; the stimulus must grow exponentially for loudness to seem to rise evenly, a principle built into volume controls today. His student Wilhelm Wundt became a founder of experimental psychology.
Helmholtz also shaped philosophy. His broadly Kantian account of how perception relates to natural law fed into neo-Kantianism late in the century. He gained the aristocratic von in 1883 and died on 8 September 1894. Germany's largest association of research institutions, the Helmholtz Association, bears his name.
Source: Hermann von Helmholtz
