Bimetallic jumping disc is a metal disc made from two different thin materials (steel and nickel) bonded together. When there is a temperature change, the two materials expand/contract at different rate. The difference in their size will cause the disc to flex suddenly. The flex will cause the disc to hit against the floor and the disc jumps into the air. This type of disc is used in equipment such as water heater, electric iron, toaster, etc to switch off the circuit when the temperature is too high or switch on the circuit when the temperature is too low.