Abstract: A sensing apparatus and method, for sensing liquids, vapors and gases, which includes a detection device constructed in accordance with the disclosures contained in U.S. Pat. No. 3,045,198, issued July 17, 1962, to Dolan et al. When the detection device is operated in a current saturated condition it becomes sensitive even to substances having a Van der Waals' "a" constant of nine or less. A change in the level of the current through the detection device occurs upon exposure of the device to the substance being sensed. This current level change is then detected to indicate the presence of the sensed substance. The relatively small current level change through the detection device may be easily discerned despite the fact that the base current is on the order of milliamperes by placing the detection device in a balanced bridge circuit. Further, the bridge circuit enables easy, accurate normalization of the apparatus in a reference environment.