The airbag "for the covering of aeroplane and other vehicle parts" traces its origins to a United States patent, submitted in 1919 by two dentists from Birmingham, Arthur Parrott & Harold Round and approved in 1920. Air-filled bladders were in use as early as 1951. The airbag specifically for automobile use is credited independently to the American John W. Hetrick, who filed for an airbag patent on 5 August 1952, that was granted #2,649,311 by the United States Patent Office on 18 August 1953. German engineer Walter Linderer, who filed German patent #896,312 on 6 October 1951, was issued on 12 November 1953, approximately three months after American John Hetrick. Hetrick and Linderer's airbags were both based on a compressed air system, either released by spring, bumper contact, or by the driver. Later research during the 1960s showed that compressed air could not inflate the mechanical airbags fast enough to ensure maximum safety, leading to the current chemical and electrical airbags. In patent applications, manufacturers sometimes use the term "inflatable occupant restraint systems".
In 1967, a breakthrough in the development of airbag crash sensors came when Allen K. Breed invented a ball-in-tube mechanism for crash detection. Under his system, an electromechanical sensor with a steel ball attached to a tube by a magnet would inflate an airbag in under 30 milliseconds. A small explosion of sodium azide was used instead of compressed air during inflation for the first time. Breed Corporation then marketed this innovation to Chrysler. A similar "Auto-Ceptor" crash-restraint, developed by the Eaton, Yale & Towne company for Ford, was soon also offered as an automatic safety system in the United States, while the Italian Eaton-Livia company offered a variant with localized[further explanation needed] air cushions. 2b1af7f3a8