A stream of successive sheets of paper, foil or the like is converted into a series of piles at a stacking station where successive sheets are caused to descend onto a pallet which is adjacent a monitoring device serving to transmit signals denoting the levels of successive uppermost sheets of a growing pile. The first light emission layer adjacent to the anode includes a host with band gaps of 1. One or more of the first andor second lateral surfaces isare arranged completely or partially with friction-enhancing means. Furthermore, since cobalt is an element having a large work function, satisfactory ohmic properties are obtained. The predetermined temperature state is defined, for example, as predetermined ranges of temperatures used during the operation and the non-operation states of the detector, respectively. When the reported symptoms exceed one of the triggers, an alert is activated to provide an indication of a potential biological or chemical terrorist attack.