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. He worked briefly on electrical systems for Chicago’s Columbian Exposition and then was hired by the E. P. Allis Company of Milwaukee.
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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