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Joseph Stefan

**Biografia** estratta http://joseph-stefan.biography.ms

**(Marzo 1835 - Gennaio 1893)** Stefan was born in an outskirt village St. Peter (Slovenian Sveti Peter) near Klagenfurt (Slovenian Celovec) in Austria-Hungary (now in Austria) to father Aleš (Aleksander) Stefan, born in 1805 and mother Marija Startinik, born 1815. His parents were married when Jožef was eleven. The Stefans were a modest family. His father was a milling assistant and mother served as a maidservant. Stefan's father died in 1872 while his mother died almost ten years earlier in 1863. Stefan attended elementary school in Klagenfurt, where he showed his talent. They recommended that he continue his schooling, so in 1845 he went to Klagenfurt gymnasium. He experienced the revolutionary year of 1848, as a thirteen-year-old boy, which inspired him to be sympathetic toward Slovenian literary production. When he had finished gymnasium as the best student in his class he thought for a while of joining the Benedictine order but he soon abandoned his idea, because his great interest for physics prevailed. He left for Vienna in 1853 to study mathematics and physics. His professor of physics in gymnasium was Karel Robida who wrote the first Slovenian physics textbook. Stefan then graduated in mathematics and physics at the University of Vienna in 1857. During his student years, he also wrote and published a number of poems in Slovenian. He taught physics at the University of Vienna, was Director of the Physical Institute from 1866, Vice-President of the Vienna Academy of Sciences and member of several scientific institutions in Europe. He published nearly 80 scientific articles, mostly in the Bulletins of the Vienna Academy of Sciences, and he is best known for originating a physical power law in 1879 stating that the total radiation from a black body is proportional to the fourth power of its thermodynamic temperature T In 1884 the law derived theoretically in the framework of thermodynamics by his student Ludwig Boltzmann and hence known as the Stefan-Boltzmann law. Boltzmann treated a heat engine with light as a working matter.

This law is the only physical law of nature named after a Slovenian physicist. With his law Stefan determined the temperature of the Sun's surface and he calculated a value of 5430 °C. This was the first sensible value for the temperature of the Sun. Stefan provided the first measurements of the thermal conductivity of gases, treated evaporation, and among others studied diffusion, heat conduction in fluids. For his treatise on optics he received the Richard Lieben award from the University of Vienna. Based on his calculations of diffusion, modern meteorologists have found a flow between the droplets of water and ice crystals, today named Stefan's flow . Very important are also his electromagnetic equations , defined in vector notation, and works in the kinetic theory of heat. He was among the first physicists in Europe who fully understood Maxwell's electromagnetic theory and one of the few outside of England who expand on it.

He calculated inductivity of a coil with a quadratic cross-section, and he corrected Maxwell's miscalculation. He also researched a phenomenon called the skin effect, where high-frequency electric current is greater on the surface of a conductor than in its interior. In mathematics the Stefan problems or Stefan's tasks with movable boundary are well known. The problem was first studied by Lamé and Clapeyron in 1831. Stefan solved the problem when he was calculating how quickly a layer of ice on water grows. He died in Vienna, Austria-Hungary.