The Ei100BNX is an Ionisation Smoke Alarm that runs on a 9V battery (supplied with the alarm). Ionisation smoke alarms operate on the principle that electrical current flowing between electrodes in an ionisation chamber is reduced when smoke particles enter. Ionisation technology gives a rapid response to fast flaming fires. The Ei100BNX is easily installed and comes supplied with all necessary screw fixings. The Ei100BNX is fitted with a warning flag that pops up when the battery is removed. This prevents the cover from closing when there is no battery fitted to the unit. When the battery is being installed or changed, this flag must be held down while gently pushing the battery into the battery holder. When the battery is first connected, the alarm may sound for 2-3 seconds ž this is normal and means that the battery is connected correctly.