Because of the thermal expansion property of metal the deformation occurs in the spring with the variation of temperature. This type of thermometer is used for measuring the ambient temperature. Spiral Strip bimetallic thermometer – In bimetallic strip thermometer, the spiral-shaped strip is used. For keeping the size of the thermometer in the manageable limit, the helix or spiral strip is used for making the thermometer. If the length of strip increases, the size of thermometer also increases. The linear strip shows the small deflection. The sensitivity of the thermometer increases with the increase of the length of the strip. This thermometer is not used for industrial application because of low sensitivity and small deflection. So, If one of the metals has the very small temperature coefficient, then the Above equation shows that if one end of the metal is fixed, the deflection of the free end of metals shows the variation of temperature. When the temperature surroundings the strip vary, the strip bends towards the circular arc.Ĭonsider the strip is made of the metal having thickness t/2. The moduli of elasticity show the ability of the material to regain its original position or shape after the removal of force or load.Ĭonsider the strip is fixed at one end and free to move at the other end. The above equation shows that the strip bend towards the metals which has a low-temperature coefficient (when the temperature increases) and the inverse will happen when the temperature decreases.įor practical applications, the strip is made of metals whose moduli of elasticity and thickness are same. The radius of the arm is given by the formula shown below. The expansion causes the strip to move in the uniform circular arc. Α A, α B– the thermal coefficient of expansion of metal A and B. T A, t B – the thickness of metal A and metal B. N – the ratio of moduli of elasticity = E B/E A Where, t – the total thickness of the strip The T 2 – T 1 shows the variation of temperature, which causes the expansion of the string. Both the metals have a different temperature coefficient. Consider the bimetallic strip is made of two different metals, i.e., metal A and metal B. Let understands this with the help of the mathematical formula. The deflection of the metal is directly proportional to the length of the strip and the variation of temperature and is inversely proportional to the thickness of the strips. The range of deflection of bimetallic strip depends on the type of metals used for construction. The strip fixed at one end and deflects at the other end. The figure below shows the bimetallic strip in the form of the straight cantilever beam. And when the temperature decreases, the strip bends towards the metal which has a high-temperature coefficient. When the temperature increases, the strip bends towards the metal which has a low-temperature coefficient. Thereby, the length of metals changes at different rates. Since the bimetallic strip of the thermometer is constructed with different metals. The physical dimension of the metals varies with the variation in temperature. The bonding is kept in such a way that there is no relative motion between the two metals. The metals are joined together at one end with the help of the welding. The bimetallic strip is constructed by bonding together the two thin strips of different metals. The expansion or contraction of metals is different at the same temperature. The temperature coefficient of all the metal is not same.The metal has the property of thermal expansion, i.e., the metal expand and contract concerning the temperature.The working principle of bimetallic thermometer depends on the two fundamental properties of the metal. Working Principle of Bimetallic Thermometer The expansion or contraction of metal depends on the temperature coefficient, i.e., at the same temperature the metals have different changes in the physical dimension. The temperature coefficient shows the relation between the change in the physical dimension of metal and the temperature that causes it. Every metal has a different temperature coefficient.
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