The marker has a housing, an inner member and a tuned circuit. The housing has a spherical cavity therein. The inner member is located inside of the housing cavity and is rotatable therein. The tuned circuit has an inductance in the form of a coil as well as a capacitance. The tuned circuit is tuned...http://www.google.com.tw/patents/US6380857?utm_source=gb-gplus-share專利 US6380857 - Self leveling underground marker
