An electronic level having a horizontal and/or a vertical transducer, each of which exhibits an effective impedance that is a function of the deviation of that transducer from a reference plane. Preferably, each transducer is an electrolytic transducer formed of a container partially filled with electrolytic...http://www.google.co.uk/patents/US4654977?utm_source=gb-gplus-sharePatent US4654977 - Electronic level
