Niels Christensen
Niels Anton Christensen (16 August 1865 – 5 October 1952) was a Danish-American inventor whose principal invention was the O-ring, the ubiquitous hydraulic seal.While working in Chicago, he witnessed an accident of a new electric railway, which resulted in two deaths and a number of injuries because the conductor was unable to stop the rail cars in time using the mechanical brake.George Westinghouse had developed an air braking system for steam locomotives and felt that Christensen's invention constituted patent infringement, suing him in December 1906.Christensen promptly countersued, starting a 24-year legal battle that went before the US Supreme Court on three separate occasions over the rights to manufacture compressed air brake systems for streetcars.In 1933, when working in his basement, Christensen discovered by trial and error that a ring-shaped piece of rubber in a groove one and a half times long as the minor radius of the ring made a reliable, tight seal of a piston sliding in a cylinder.