Abstract: A fire sensing unit is provided with apparatus which sends out a code identifying the type of sensor apart from transmission of the quantized signals of the analog output of the sensor of the sensing unit. Thus the data processing unit of the receiving unit of a fire detection system including the sensor can be simplified because the receiving unit need not collate the types of the installed sensors with a list of addresses stored therein. Also replacement of sensors can be easily effected since the information regarding the type of sensors is stored in the sensing unit.

Abstract: An ionization smoke detector having two ionization chambers with one of said chambers having an electrode at least partially surrounded by a conductive mesh constituting an intermediate electrode and the other of said chambers being formed between the intermediate electrode and an outer surrounding electrode of conductive mesh and a radioactive source carried by said outer electrode. The volume defined by the outer and intermediate electrodes is greater than the volume defined by the intermediate electrode.