Marcel Deprez (29.12.1843- 16.10.1918) was a French physicist and engineer and a member of the Paris Academy of Sciences (1886). He was born in Aillant-sur-Milleron. He graduated from the École des Mines in Paris (1866) and in 1866—72 was secretary to the director of that school. From 1890 he was professor at the Conservatoire des arts et métiers (Paris).
His main research was in the field of electromagnetism and electrical engineering. He invented the absolute electrodynamometer, the wattmeter and the ammeter, and together with J. D’Arsonval the aperiodic galvanometer (1882). He improved the dynamo and discovered the principle of compound excitation (compounding).
In 1880, independently of D. A. Lachinov, he showed theoretically that electric power can be transmitted over long distances without considerable loss provided that the voltage is raised. In 1881 he built a direct-current transmission line 35 miles long from Miesbach to Munich. Later he built a number of such lines in France. He did much for the use of the energy of electric current in industry. He also produced an instrument for determining the speed of a shell in the barrel of a gun and a series of instruments for dynamometric measurements on railway track. He studied the pressure of gases, the deformation of metals, the laws of friction and the mechanical equivalent of heat.