This is a door wedge intruder alarm and despite looking simple would be quite effective in warning you if somebody was trying to get into your room through the door. It consists of what looks like bicycle bell which is fixed to a flat piece of metal with a wedge of metal on the other end. Protruding from the wedge is a small metal pin, which you can just see if you look closely at the photographs.
It works like this; The bell dome is turned to wind up a mechanism inside the bell, just like winding up an old alarm clock; you then gently push the wedge end under the door so that it fits snug. If the door is forced open the wedge closes and pushed the pin into the bell releasing the mechanism inside and a little hammer hits the inside of the dome repeatedly giving a constant ringing sound, just like a bicycle bell.
This wind-up wedge door intruder alarm is pre.1908 and was produced by British Automatic Alarm Bell Co Ltd it was patented in Britain but made in Germany The whole thing is no more than 4 inches (10 cm) long. These types of alarms were often carried by travelers who regularly stayed in hotels in the days when hotel bedroom doors were not as secure as they usually are today. It seems they couldn’t be to careful.
When researching this door wedge intruder alarm I realized that although the original design of these items goes back to the 1870s there are very similar ones to this still being manufactured today. similar door wedge alarms are still being manufactured but they are now battery operated and emit a high pitched electronic alarm sound instead of a bell ringing sound.