Door knob burglar alarm

Claim 1

An alarm device for placement on a door knob or the like, comprising: a. an alarm housing having a receptacle formed therein, the receptacle being sized to fit over a door knob and lined at least in part with a predetermined thickness of resilient, material disposed for frictional engagement with the door knob to permit movement of the alarm housing with rotational motion of the door knob; b. electric circuit means disposed within the alarm housing and comprising 1. battery means; 2. annunciator means; 3. first and second mercury switch means for closing the circuit between the battery means and the annunciator means, each of said mercury switch means occupying a normally open position with the alarm housing in a no-alarm position; 4. the first mercury switch means being disposed for closing movement in response to rotation of the housing in a first direction; 5. the second means being disposed for closing movement in response to rotation of the housing in an opposite direction; 6. each of said mercury switch means being angularly adjustable to determine the rotational position at which it closes; 7. and an on-off switch disposed to override the first and second mercury switch means and render said annunciator means inoperable.