Expansion and Contraction
Thermo-fluids Thermal Expansion and Contraction (*)
The principle behind a bimetallic strip is that different metals expand to different extents with temperature changes. By combining two different metals one on top of another into a strip, a bimetallic strip is formed. As the two metals expand or contract differently under the same temperature change, the strip bends. It can then be used to switch on or off a circuit at certain temperatures. Bimetallic strips are often found in ovens. The typical structure of this type of control is shown in Fig. 5.
|Fig. 4 A typical bimetallic strip||Fig. 5 The structure of a bimetallic strip|
The device shown in Fig. 5 is typical of those used in ovens. The upper metal (blue) expands more when heated and contracts more when cooled than the lower metal. Thus, when the temperature inside the oven drops below a certain point, the bimetallic strip bends upwards enough to complete the circuit, switching on the heating element. In a refrigerator, the reverse set-up is used. When the temperature inside the refrigerator rises, the bimetallic strip bends to switch on the compressor which starts the cooling cycle.