Heinrich Rudolf Hertz (22.02.1857-1.01.1894) was a German physicist and a corresponding member of the Berlin Academy of Sciences (1889). He was born in Hamburg. He graduated from the University of Berlin (1880) and was an assistant to H. Helmholtz. In 1885—89 he was professor at the Higher Technical School in Karlsruhe, and from 1889 at the University of Bonn.
His main work was in electrodynamics, of which he is one of the founders, and in mechanics. In 1887, in the paper “On Very Rapid Electric Oscillations”, he proposed a successful design for a generator of electromagnetic oscillations (the Hertz vibrator) and a method of detecting them by means of resonance (the Hertz resonator), thereby developing for the first time the theory of the open vibrator that radiates electromagnetic waves into space. Using the vibrator and the resonator, in 1888 he proved experimentally the existence of electromagnetic waves propagating in free space, which had been predicted by Maxwell's theory. Experimenting with electromagnetic waves, he observed their reflection, refraction, interference and polarisation. He established that the speed of propagation of electromagnetic waves is equal to the speed of light. Hertz's experiments were of great importance for the acceptance of Maxwell's theory and its establishment. Extending Maxwell's theory, he gave (1890) the equations of electrodynamics a symmetrical form which brought out clearly the complete interconnection between electric and magnetic phenomena (the Maxwell — Hertz electrodynamics). In 1887 he observed the external photoelectric effect, noticing that the electric discharge between two electrodes is stronger (or occurs at a lower voltage) if the spark gap (the electrodes) is illuminated by light rich in ultraviolet rays.
Starting from the hypothesis that the ether is completely carried along by moving bodies, in 1890 he constructed a general theory of electromagnetic phenomena in moving bodies (the electrodynamics of moving bodies). Hertz's electrodynamics, however, contradicted some experiments and was in time replaced by Lorentz's electron theory.
Hertz's research also concerned cathode rays, the theory of the impact of elastic bodies and so on. In the paper “On the Passage of Cathode Rays through Thin Metal Layers” (1891) he discovered the transparency of metals to cathode rays, thereby laying the foundations for the study of cathode rays and of the structure of matter. He constructed a mechanics free of the concept of force which, although it was interesting, proved of little fruitfulness (the introduction of non-holonomic constraints, the treatment of a mechanical system as a system of many dimensions with a large number of degrees of freedom, and the principle of the shortest path, or of least curvature — Hertz's principle).