Abstract: An intrusion detecting switch is disclosed having an operator and a sensing device for sensing the detection of intrusions and to a protected area. The operator may be a magnet and the sensing device may be Hall effect switches the resistance of which will switch between high and low values in the presence and absence of the magnetic field. A mounting device may be provided for mounting the housing containing either the sensing device or the operator to its support. The mounting device is located behind the operator or sensing device such that if the housing is to be removed from its support, the sensing device or operator must be first removed which will result in an alarm.