This section is from the book "The Boy Mechanic Vol. 2 1000 Things for Boys to Do", by Popular Mechanics Co.. Also available from Amazon: The Boy Mechanic, Vol2: 1000 Things for Boys to Do.
A simple door-bell alarm for informing one when the door of a shop or dwelling is opened is shown in the accompanying sketch. It consists of a piece of spring brass, A, bent into a circle in the center so that it may be clamped on the doorknob bar by means of a small bolt or screw. The two ends of this piece should be separated as shown and a second piece, B, mounted on the door so that its outwardly projecting end is between the ends of the piece A. One terminal of an ordinary vibrating bell circuit is then connected under the head of the clamp screw, and the other terminal under one of the screws holding the piece B in place on the door. It is now obvious that the bell circuit will be completed and the alarm sounded when the knob is turned. Make sure that the piece A is bent so that the circuit is completed before the latch has moved a sufficient amount to allow the door to open.
Ill: Wiring Diagram and Connections to an Electric Bell That Rings When a Door Knob is Turned
The circuit leading to and from the switch may be completed through the hinges of the door, but it would be better to use small coil springs as shown. There would then be no likelihood of the circuit being open at any time, which might occur if the hinges were used.
CDiscolored coffee and teapots may be restored to their original brightness by boiling them a few minutes in a solution of borax water.