The Bimetallic Strip  
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The bimetallic strip consists of two different metals that expand at different rates when heated  
through the same temperature change.  
The bending of the bimetallic strip is due to the difference in the coefficient of linear  
expansion of two different metals used in the bimetallic strip.  
A bimetallic strip consists two strips of different metals fixed to each other lengthwise. An  
increase in temperature causes bending of the strip in such a way that the metal of greater  
linear coefficient of expansion lies on the outer side.  
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A lowering of temperature again bends the strip, but with the metal of smaller linear  
coefficient of expansion on the outer side. Such strips can be used in electric iron, electric  
oven, refrigerator to control the temperature.  
Applications of expansion of Solids  
1. Applied in the construction of bridges and house roofs  
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Iron or steel girders are used in the construction of a bridge. One end of the girder is rigidly  
fixed with bricks and concrete. The other end is not fixed. Instead, the end is set on a roller  
over the support, as shown in Fig. When there is rise or fall in temperature due to seasonal  
changes, girders may expand or contract, without developing any thermal stress.  
2. Railway lines construction  
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The rails of a railway expand in summer and contract in winter. Therefore during construction  
gaps are left between successive rails to allow for their expansion. If there are no gaps, the  
increase in temperature will cause the rails to expand and they will overlap one another or  
dislodge from the position. This will be dangerous to the trains and may result in a severe  
accident.  
3. Construction of roads and pavements  
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While constructing cement roads using concrete slabs, a small gap is left between two  
slabs. The concrete slabs undergo thermal expansion during summer. The gap allows  
space for this expansion. If these gaps were not left, the concrete slabs would crack during  
summer due to thermal expansion. (See the fig below)  
4. The use of bimetallic strip  
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Bimetallic strips are used as temperature controlling devices.  
The bimetallic strip is used in thermostats, thermometers and valves.  
(a) In Thermostat: Is a device used for maintaining a steady temperature.  
Thermostat is used in many appliances such as electric irons, heaters, refrigerators, air  
conditioners, fire alarms and Valves. Thermostat is an automatic switch that turns on at a  
definite temperature and off at some other fixed temperature.  
Fig. above shows the working of a thermostat.  
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The two metals in the bimetallic strip are brass and invar. The contact point  
thermostat is kept adjacent to the brass part of the bimetallic strip.  
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of the  
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At ordinary temperature the metal strip remains straight. This keeps the thermostat switched  
on and current flows through the heater. Heat from the heater warms up the air. The strip  
gets heated up and bends. Coefficient of linear expansion of brass is more than that of invar  
and so the bimetallic strip bends away from the contact point  
stopping current flow through the heater.  
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and disconnects the circuit,  
Hence, temperature of the heater falls. This also cools the bimetallic strip. The strip  
straightens up again restoring the electrical connection, and the heater is on again.  
Therefore, the heater cannot remain ‘ON’ above a fixed temperature as the thermostat  
controls the current flow. Thus, the temperature is also controlled.  
Summary of working mechanism;  
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When a thermostat is used in an electrical circuit, its bimetallic strip gets heated and bends if  
the temperature in the circuit exceeds a certain limit. Hence the circuit breaks. Since there is  
no current in the circuit now, the temperature of the strip decreases. The strip gets restored to  
its original position, and the circuit is completed, allowing current to flow again.  
In Fire alarms circuit  
A bimetallic strip of brass and iron is connected to a battery and an electric bell as shown in  
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the diagram. One terminal of the bell is connected to a screw which is at a very small  
distance from the iron side of the strip. In case, there is an accidental fire, the bimetallic strip  
bends towards iron and touches the screw. Thus, the circuit is completed and the bell rings,  
thereby warning the people of the accidental fire.  
In Electric iron  
In an electric iron (see Figure below) when electric current flows through its heating element,  
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it becomes hot. The bimetallic strip connected with the heating element through a spring also  
begins to heat up. On getting hot, bimetallic strip bends and is disconnected from the heating  
element. This makes the circuit open and switches OFF the electric iron. On cooling, the  
bimetallic strip straightens. The circuit is again closed and the iron is switched ON.  
(b) In Bimetallic thermometer  
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The figure below shows a spiral bimetallic thermometer instrument. The spiral bimetallic strip  
is attached to the pointer mounted in housing with a scale.  
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When the spiral bimetal is heated, it bends in the direction of metal having low thermal  
coefficient of expansion, and rotates in clockwise direction. Thus, the pointer attached to it  
also moves on calibrated scale, indicating the temperature reading.  
(c) In Bimetallic Valve  
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Is also used to open and close temperature – sensitive valve  
The valve opens when the temperature rises and close when the temperature falls  
5. Applied in hot-water pipes  
In certain industries, hot liquids or hot water is transported through metal pipes from one  
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place to another. These pipes are subjected to expansion and contraction. To avoid cracks  
in the pipes when they expand and contract, they are arranged in the form of loops.  
6. Applied in designing of pendulum clocks  
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The pendulums of clocks expand in summer and contract in winter, therefore, they lose  
time in summer and gain time in winter. In order that the clocks should give correct time,  
the pendulums are made from invar which is an alloy having a very small coefficient of  
linear expansion in some clocks, compensating pendulums are used.  
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A compensating pendulum is made of a number of iron and brass rods joined in such a way  
that the length of the pendulums remains constant even if there is a change in temperature.  
Such clocks show accurate time. If this is not possible the clocks require the adjustment in  
summer and winter.  
7. Overhead telephone and electrical cables are loosely held  
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In order to allow for contraction and expansion during the cold and hot day respectively  
During cold weather or at night, the metal can contract without breaking  
Some effects of expansion and contraction  
1. Vessels made of thick or ordinary glass break easily  
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A thick walled glass pot often cracks when hot water is poured into it. The inner surface  
of the thick glass, in contact with the hot water, warms up and expands. Glass is a bad  
conductor of heat, and so the outer surface remains colder, and hence expansion is  
less. This unequal expansion sets up a thermal stress and the glass cracks.  
In case of a thin walled glass pot, heat transfer to the outer surface is easier and there is  
almost equal expansion of both the surfaces. The chance of cracking is reduced. Pyrex  
glass has a low coefficient of expansion. Hence, beaker, test tube, etc. for laboratory use  
are usually made of pyrex glass.  
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2. Riveting of two metal plates  
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Rivets are used in shipbuilding, boiler making, etc to join metals sheets  
When ships and boilers are constructed, the steel plates are joined together firmly by  
riveting. This is also an application of thermal expansion and contraction. A rivet is  
heated to red hot and passed through the plates. Then it is hammered to fix it firmly. On  
cooling, the rivet contracts and holds the plates together firmly  
3. Glass stoppers sometime stick in the necks of bottle  
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By warming the neck of such a bottle gently, the stopper often comes out easily. This is  
because the neck of the bottle expands when hot water is poured over it. As a result, a  
small gap is created between the stopper and glass, enabling the easy removal of the  
stopper.  
4. Weathering of rocks  
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This is the action of sun, air and water on rocks, causing them to break  
Hot sun shine makes the outside of a rock expand, and pieces break off due to the force  
of expansion  
5. To Pass a Nail Through Hole in Metal Plate:  
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To pass nail through a metal plate having a hole of a diameter slightly less than that of the  
nail, the plate is heated. so that the diameter of hole increases and the nail can easily pass  
through it.  
6. Use of thick Bottles for Soft drinks:  
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To avoid bursting of soft drink bottles containing gas, due to thermal expansion, their walls  
are made very thick.  
7. Fitting the iron rim on the wooden wheel:  
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The iron ring is always made slightly lesser in diameter than that of the wooden  
wheel. Therefore, the iron ring cannot easily slip onto the rim of the wooden wheel.  
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Now the iron ring is heated to a higher temperature, it results in the expansion in the  
size of the ring, and the hot ring is then easily slipped onto the wooden wheel's rim.  
Cold water is then poured on the iron ring to contract in size and hold the wooden  
wheel tightly  
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Class Activity – 6:3  
1. Explain why  
(a) The metal lid on a glass jam jar can be unscrewed easily if the jar is inverted for a few  
seconds with the lid in very hot water,  
(b) Furniture may creak at night after a warm day,  
(c) Concrete roads are laid in sections with pitch between  
2. Why is an iron rim heated before fixing a cart wheel?  
3. Give a reason why a concrete beam reinforced with steel does not crack when subjected to  
changes in temperature  
4. A blacksmith fixes iron ring on the rim of the wooden wheel of a bullock cart. The diameters of the  
rim and the iron ring are 5.443 m and 5.434 m respectively at 370C. To what temperature the iron  
ring should be heated so that it can fit the rim of the wheel. (Given that  
∝
for iron = 1.20 x 10-5K  
)
ANS: 1750C  
5. Brass expands more than invar when heated equally. Sketch the bimetallic strip after being  
cooled several degrees below room temperature.  
6. Railway lines 25m long are being laid when the temperature is 150. Gaps of 1.8cm are being left  
between them. If  
= 12x10-6 0C-1, what is the temperature at which they will touch each other?  
[ANS; 750C]  
7. A bimetallic strip is made from aluminium and copper. When heated it bends in the direction  
shown in Figure below  
(d) Which metal expands more for the same rise in temperature, aluminium or copper?  
(e) Draw a diagram to show how the bimetallic strip would appear if it were cooled to below  
room temperature.