Much of the information in these articles comes from the CIA World Factbook 2000 and the 2002 U.S. Department of State website. Somewhat Wikified.
... the slipping friction. The device is covered by US patents 3,726,146 (1973) and 5,353,655 (1994) by L.A. Mishler and US patent 5,800,311 (1998) by ...