Why does the disc jump?
The jumping disc works on the same principle as a thermostat or a shunt-valve in your car. With such a thermostat or automatic valve, a special piece of metal, called bi-metal, moves when heat is applied or removed. Bi-metal is actually two pieces of thin metal fused together in the middle. When the temperature changes, the metals expand differently and bend into a curve in either direction.

How it works!
The jumping disc is made from a special piece of metal, called bi-metal that moves when heated with your thumb. When it is warmed to about body temperature, you can "click it" into its loaded position and then carefully place it on a hard surface. When it cools down to room temperature, it suddenly snaps back into the old position, and simultaneously jumps high up in the air. It jumps several hundred times its own height.

Thermostatic bimetal types
The type of thermostatic bimetal depends on the application and the available space. Elements can be roughly classified into four main groups:
Beams,
Coils,
Discs,
Formed elements.
Beams develop nearly linear motion. Coils (spiral, helical and double-helix coils) Disks move linearly or discontinuously (snap-action).
Formed elements include a wide range of regular or irregular shapes. The U-shape is very widespread. This type of element is employed when the available space will not permit the use of long, flat beams.

Generally speaking, coils are used wherever extensive motion is required, while beams are required for smaller movements to trip operating mechanisms.

some basic mechanism of bimetal strip :
1.Simple temperature -controlled switch
This type of mechanism is usd for drect switching of an electrical circuit or a mechanical part. When current is passed directly through the element , slow contact motion may result in sparking between the contacts.
2.Counter-acting arrangements
Faster response and higher sensitivity reduces the risk of sparking between contacts in elements with direct electrical heating .
3.Auxiliary thermal switches
The thermostatic bimetal does not form part of the electrical circuit and responds solely to the ambient temperature .
4.Magnetic restraint
a permanent magnet resistrains deflection until the thermostatic bimetal had developed a force sufficient to overcome that of the magnet ,thus causing the contacts to spring parts . Conversely , the contacts close when the element has cooled down.