|Niels Anton Christensen|
August 16, 1865|
|Died||October 5, 1952|
|Profession||Engineer and Inventor|
Niels Anton Christensen was born on a farm in Tørring-Uldum Municipality, Denmark. He showed an early aptitude for mechanics and apprenticed to a machinist in Vejle, Denmark. After completing his apprenticeship, he entered the Technical Institute of Copenhagen, now the University of Copenhagen Faculty of Science. In 1891, Christensen immigrated to the United States when he was 26 years old.
Christensen became a leading draftsman at Fraser and Chalmers in Chicago, a manufacturer of machinery for industry, mining, and transportation. 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. Christensen decided to design and build a power brake. The Christensen air brake was successfully tested on Detroit's streetcar system, but a downturn of the economy prevented Christensen from manufacturing and marketing the system.
He worked briefly on electrical systems for Chicago’s Columbian Exposition and then was hired by the E. P. Allis Company of Milwaukee. While at E. P. Allis, he continued to develop his air brake for electric rail cars and streetcars. In 1896 he obtained financial backing that allowed him to make an experimental test apparatus. He also secured patents on the new valve mechanism. In early 1897, he founded Christensen Engineering Company, which initially operated in the Menomonee Valley at 718 Hanover. His operations were co-located with the Seamless Structural Company at the corner of Hanover and Burnham Streets.
In December 1906, Westinghouse sued for patent infringement. George Westinghouse, had developed an air braking system for steam locomotives and he didn’t take lightly what he considered an illegal use of his patent rights. Westinghouse Air Brake Company acquired National Electric, renamed the company the National Brake & Electric Company, which refused to pay royalties to Niels Christensen. 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, 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. He applied for a U.S. patent in 1937 and it was granted two years later.
After Pearl Harbor, the United States government bought the rights to many war-related patents, and made them available to manufacturers royalty-free. Christensen was paid $75,000. When the war ended (formally in 1952) and the patent rights were transferred back to him, the patent had only four years left. Litigation resulted in a $100,000 payment to his heirs in 1971, 19 years after his death. 
- No. 555:O-Ring (John H. Lienhard, University of Houston)
- O-Rings: A Public Radio Commentary by Bill Hammack (William S. Hammack Enterprises) 
- O-Ring History (Ge Mao Rubber Industrial Co., Ltd.)
- Ring Master (George Wise, "Invention and Technology Magazine" Spring/Summer 1991, Volume 7, Issue. American Heritage Publishing 
- 339 F2d 665 Jo. C. Calhoun, Jr., and Esther C. Young, Executors of the Estate of Niels A. Christensen (Deceased) v. United States (OpenJurist.org) 
- Weber, Robert John and David N. Perkins Inventive minds: creativity in technology (Volume 49. Oxford University Press. 1992)
- Christensen, Niels A. U. S. Patent 2,180,795 (Packing. November 21, 1939)
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