Abstract: A gas detector for sensing the presence of a gaseous medium consisting of a helical coil for connection to a dc power source for generating both an electromagnetic field and radiant heat. The coil is wrapped around a tubular sleeve constructed of a sensing material, such as a semiconductor, and disposed in the magnetic and heating fields of the coil. An electrode is disposed in the sleeve so that when a current sensing medium is connected between the electrode and one end of the coil, a current will be produced proportional to the concentration of the gas being detected. In several embodiments, the sleeve is constructed of an n-type semiconductor material, such as zinc oxide, stannic oxide, or gallium oxide and is good for detecting hydrocarbons. If the sleeve is constructed of tungsten trioxide or molybdenum trioxide mixed with silicon dioxide, the sleeve can be used for detecting hydrogen. The sleeve is generally operated at temperatures between 100.degree. and 450.degree. C.

Abstract: A coulometer signaling device is disclosed which indicates when a particular machine has been used for a predetermined amount of time. The device comprises a mercury coulometer which includes an electrolyte whose color changes when the mercury deposited on the anode of the coulometer has been depleted. The end of the tube near the coulometer is somewhat enlarged and a lens is placed over this end in order to make the color change more visible.