this intrusion alarm apparatus is intended to be suspended from an inside doorknob and plugged into an a . c . power socket at the premises to be protected . a test switch and indicator lamp enable the user to determine whether the plug has the correct polarity in the power socket . the alarm signalling device in the apparatus or the indicator lamp is activated by the added capacitance of a person or an extraneous metal object touching the outside doorknob , depending upon whether the switch is in the &# 34 ; test &# 34 ; or &# 34 ; on &# 34 ; position .