Hermann von Helmholtz

Helmholtz trained as a military surgeon in Berlin — the state paid for his medical school in exchange for eight years of army service — but his real interests were in physics and physiology. His 1847 essay Über die Erhaltung der Kraft (On the Conservation of Force) was the first rigorous mathematical statement of energy conservation, welding together Joule's experiments, Mayer's speculations, and Clausius's thermodynamics into a single principle.

He then turned to the physics of perception. His three-volume Handbook of Physiological Optics (1856–67) laid the foundations of modern vision science. His 1863 On the Sensations of Tone did the same for hearing, combining Fourier decomposition of complex waveforms with anatomy of the inner ear to explain why a violin and a clarinet playing the same note sound different. The book became the standard text on acoustics for more than a century.

He held chairs at Königsberg, Bonn, Heidelberg, and finally Berlin. His students included Heinrich Hertz — who would go on to discover radio waves — and Max Planck, who inherited his Berlin chair. When the Reichsanstalt, the German national standards institute, was founded in 1887, Helmholtz was its first president. He was the pre-eminent scientist of late-nineteenth-century Germany, and one of the last figures to bridge physics, physiology, and philosophy in a single career.