704,463 for Hydraulic brakes, filed December 29, 1933, now U. S. Patent No. 2,115,383 granted April 26, 1938". Soon after migrating to the United States in 1891, he patented an air brake system for streetcars (trams). Despite his legal efforts, the patents were passed from company to company until they ended up at Westinghouse. During World War II, the US government commandeered the O-ring patent as a critical war-related item and gave the right to manufacture to other organizations. Christensen received a lump sum payment of US$75,000 for his efforts. Litigation resulted in a $100,000 payment to his heirs in 1971, 19 years after his death. Theory and design O-rings are available in various metric and inch standard sizes. Sizes are specified by the inside diameter and the cross section diameter (thickness). In the US the most common standard inch sizes are per SAE AS568C specification (e.g. AS568-214). ISO 3601-1:2012 contains the most commonly used standard sizes, both inch and metric, worldwide. The UK also has standards sizes known as BS sizes, typically ranging from BS001 to BS932. Several other size specifications also exist. Typical applications Successful O-ring joint design requires a rigid mechanical mounting that applies a predictable deformation to the O-ring. This introduces a calculated mechanical stress at the O-ring contacting surfaces. As long as the pressure of the fluid being contained does not exceed the contact stress of the O-ring, leaking cannot occur. The pressure of the contained fluid transfers through the essentially incompressible O-ring material, and the contact stress rises with increasing pressure. For this reason, an O-ring can easily seal high pressure as long as it does not fail mechanically. The most common failure is extrusion through the mating parts. The seal is designed to have a point contact between the O-ring and sealing faces. This allows a high local stress, able to contain high pressure, without exceeding the yield stress of the O-ring body. The flexible nature of O-ring materials accommodates imperfections in the mounting parts. But it is still important to maintain good surface finish of those mating parts, especially at low temperatures where the seal rubber reaches its glass transition temperature and becomes increasingly inflexible and glassy. Surface finish is also especially important in dynamic applications. A surface finish that is too rough will abrade the surface of the O-ring, and a surface that is too smooth will not allow the seal