**James Clerk Maxwell (1831-1879)**

**is regarded by most modern physicists as the scientist of the 19th century who had the greatest influence on 20th-century physics; he is ranked with Sir Isaac Newton and Albert Einstein for the fundamental nature of his contributions. In 1931, at the 100th anniversary of Maxwell’s birth, Einstein described the change in the conception of reality in physics that resulted from Maxwell’s work as “the most profound and the most fruitful that physics has experienced since the time of Newton.” The concept of electromagnetic radiation originated with Maxwell, and his field equations, based on Michael Faraday’s observations of the electric and magnetic lines of force, paved the way for Einstein’s special theory of relativity, which established the equivalence of mass and energy. Maxwell’s ideas also ushered in the other major innovation of 20th-century physics, the quantum theory. His description of electromagnetic radiation led to the development (according to classical theory) of the ultimately unsatisfactory law of heat radiation, which prompted Max Planck’s formulation of the quantum hypothesis– i.e., the theory that radiant-heat energy is emitted only in finite amounts, or quanta. The interaction between electromagnetic radiation and matter, integral to Planck’s hypothesis, in turn has played a central role in the development of the theory of the structure of atoms and molecules. **

**During the most fruitful of his career. this period his two classic papers on the electromagnetic field were published, and his demonstration of colour photography took place. He was elected to the Royal Society in 1861. His theoretical and experimental work on the viscosity of gases also was undertaken during these years and culminated in a lecture to the Royal Society in 1866. He supervised the experimental determination of electrical units for the British Association for the Advancement of Science, and this work in measurement and standardization led to the establishment of the National Physical Laboratory. He also measured the ratio of electromagnetic and electrostatic units of electricity and confirmed that it was in satisfactory agreement with the velocity of light as predicted by his theory. **

**It was Maxwell’s research on electromagnetism that established him among the great scientists of history. In the preface to his Treatise on Electricity and Magnetism (1873), the best exposition of his theory, Maxwell stated that his major task was to convert Faraday’s physical ideas into mathematical form. In attempting to illustrate Faraday’s law of induction (that a changing magnetic field gives rise to an induced electromagnetic field), Maxwell constructed a **

**mechanical model. He found that the model gave rise to a corresponding “displacement current” in the dielectric medium, which could then be the seat of transverse waves. On calculating the velocity of these waves, he found that they were very close to the velocity of light. Maxwell concluded that he could “scarcely avoid the inference that light consists in the **

**transverse undulations of the same medium which is the cause of electric and magnetic phenomena.” **

**Maxwell’s theory suggested that electromagnetic waves could be generated in a laboratory, a possibility first demonstrated by Heinrich Hertz in 1887, eight years after Maxwell’s death. The resulting radio industry with its many applications thus has its origin in Maxwell’s publications. **

**The Maxwell relations of equality between different partial derivatives of thermodynamic functions are included in every standard textbook on thermodynamics ( THERMODYNAMICS, PRINCIPLES OF). Though Maxwell did not originate the modern kinetic theory of gases, he was the first to apply the methods of probability and statistics in describing the properties of an assembly of molecules. Thus he was able to demonstrate that the velocities of molecules in a gas, previously assumed to be equal, must follow a statistical distribution (known subsequently as the Maxwell-Boltzmann distribution law). In later papers Maxwell investigated the transport properties of gases– i.e., the effect of changes in temperature and pressure on viscosity, thermal conductivity, and diffusion. **