Alexander Grigoryevich Stoletov (29.07.1839-14.05.1896) was a Russian physicist. He was born in Vladimir. He graduated from Moscow University (1860) and was kept on to prepare for a professorship. In 1862—66 he furthered his studies under H. Magnus, G. Kirchhoff and W. Weber. After returning from his assignment abroad he worked at Moscow University (professor from 1873).
His scientific work concerned electromagnetism, optics, molecular physics and philosophical questions of science. In his doctoral dissertation “Studies of the Function of Magnetisation of Soft Iron” (1872) he showed for the first time that as the magnetising field increases, the magnetic susceptibility of iron at first rises and then falls, passing through a maximum. He was the first to record the curve of the magnetic permeability of a ferromagnet (the Stoletov curve). He proposed two important methods of magnetic measurement of substances — the method of a toroid with a closed magnetic circuit, and the ballistic measurement of magnetisation. He carried out a number of experiments to determine the ratio of electromagnetic to electrostatic units, and obtained a value close to the speed of light (1876).
In 1888—90 he carried out a series of studies of the external photoelectric effect, detected by H. Hertz in 1887. In 1888, independently of W. Hallwachs and A. Righi, he rediscovered this effect, created the first photocell based on the external photoelectric effect and applied it in practice, considered the question of the inertia of the photocurrent and estimated its lag behind the illumination at 0.001 s. He discovered the direct proportional dependence of the photocurrent on the intensity of the incident light (the first law of the external photoelectric effect) and the phenomenon of photoelectric fatigue — the decrease of the sensitivity of a photocell with time (1889), detected the saturation photocurrent and showed its independence of potential (1890). He was the founder of quantitative methods of studying the photoelectric effect and proposed a method of photoelectric control of the intensity of light.
He studied the non-self-sustained gas discharge and established that the ratio of the electric field strength to the gas pressure at maximum current is a constant (the Stoletov constant). He studied the critical state of matter (1892-94).
He did much for the development of physics in Russia. In 1872 he organised the first physics laboratory, and he initiated the creation of a physics institute at Moscow University. He was the author of a number of philosophical essays, in which he spoke as a materialist, and also of essays on the history of science. He trained many Russian physicists (D. A. Goldhammer, P. A. Zilov, N. P. Kasterin, R. A. Kolli, V. A. Mikhelson, A. P. Sokolov, B. V. Stankevich, N. N. Schiller, V. S. Shcheglyaev and others).