The working of bimetals is based on the theory that a metal expends on heating and contracts on cooling. If we consider a strip of metal of length of l. When the temperature increases, it length increased. The increase in length of strip due to rise in temperature is related by Coefficient of linear thermal expansion. It denoted by “αL”.
Where, l is the initial length of object,
Δl is the rise in length,
Δt is the rise in the temperature,
Unit of αL is per oC.
Bimetal is consists of two strips of two different metals having different Coefficient of linear thermal expansion, welded together lengthwise. A bimetal at normal temperature is shown in figure below.
On heating, the expansions in the length of both metal strips are different. Due to which the bimetallic element bends and form an arc in such a way that the metal with higher Coefficient of linear thermal expansion is outer at side of the arc and metal with lower Coefficient of linear thermal expansion is at inner side of the arc as shown in figure below.
On cooling, bimetal element bends and form an arc in such a way that the metal with lower Coefficient of linear thermal expansion is at outer side of the arc & metal with higher Coefficient of linear thermal expansion is at inner side of the arc as shown in figure below.
The above phenomenon can used to produce a useful device for detecting and measuring change in temperature.
Commonly Used Combinations for Making Bimetallic Strips
Many combinations of metals with different Coefficient of linear thermal expansion can be used to form the bimetals. Some of the commonly used combinations for making bimetallic strips are listed below-
- Iron, nickel, constantan (high “Coefficient of linear thermal expansion”)
- Alloy of iron and nickel (low “Coefficient of linear thermal expansion”)
Applications of Bimetals
Bimetal are having many applications. Some of these are listed below-
Thermostat of Bimetals
Bimetals are very much useful to make the bimetallic thermostat for automatic switching of circuit to control the temperate of certain appliance such as, Electric heater, Electric irons, refrigerators, electric ovens etc. In some circuits, the current passed through the thermostat itself produce the heat for operation of thermostat. A typical Bimetallic thermostat for this operation is shown in figure below-
In operation as a thermostat, one end of bimetal is fixed and connected to source of supply. Other end is free to move. An electrical contact is connected on free end of bimetal, which moves with the expansion and bending of bimetal. At normal temperature, this moving contact makes the contact with fixed contact as shown in figure below. When the heat is produce this bimetallic strip bends. Due to which the contact attached with the free to move end of bimetallic strip moves and disconnects from fixed contact as shown in figure below. Again when the temperature decrease and come back to normal condition, the bimetal comes back the normal condition and as the circuit is closed by bimetal. An example of this type of thermostat is thermostat used in electric iron.
In above figure, the bimetal is a straight strip, however it can be made in the form of a coil (as shown in figure below) to increase the length in order to increase the sensitivity of thermostat.
Clock of Bimetals
Mechanical clock system is very much sensitive to temperature which results in error in operation of watch. By using bimetallic strip, the error can be compensated up to a considerable extent.
Thermometer of Bimetals
Bimetallic strip can be used in direct indicating type thermometers. In thermometer, the bimetal strip is used in coil form. A typical type of thermometer using bimetallic strip in coil form is shown in figure below –
One end of this coil is connected to the housing of instrument and the other end is connected to the pointer. On heating of bimetallic strip coil, the free end of coil moves which results in deflation of pointer on scale. Scale is calibrated in term of temperature. So that the deflection of pointer directly indicates the temperature on scale.
Electrical protective devices of Bimetals
Bimetallic thermostat based relays are used for detecting the over current in electric devices to protect them from over current. The current flowing through circuit or device is passed througha heating coil which heats the bimetallic strip. Due to heating the bimetallic strip bends on operate the tripping circuit to protect the electrical circuit or device from over current.