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.
The falling temperature of a room during the night may result in a very bad cold for the occupant. This may be prevented by the use of an alarm to awaken the sleeper and warn him to close the window. An alarm can be made as follows: Take a glass tube about 4 in. long and 1/4 in. in diameter and close one end, used for the bottom, with sealing wax, in which the bare end of a No. 20 gauge magnet wire is inserted. The tube is almost filled with mercury. On the mercury a float of wax is placed in which a bare piece of the same magnet wire is inserted and bent as shown in the sketch. The tube of mercury is fastened to a base with two clips of metal. At the upper end of this base the adjustable lever A is attached. The electric connections are made as shown in the sketch.
Ill: The Alarm and Wiring Diagram
Should the temperature fall during the night, the mercury will contract, the float descend and the circuit close, so that the bell will ring. The adjustable lever allows setting the alarm for various differences of temperature. - Contributed by Klyce Fuzzelle, Rogers, Ark.