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Describe a method to determine the linear expansivity of a solid. Can the cubical expansivity be determined from this value?

Determination of coefficient of linear expansion  of a solid material by using Pullinger’s Apparatus :

 The coefficient of linear expansion is determined by using Pullinger’s apparatus. It comprises of solid material, hollow cylinder, thermometer, heat supply kit and a simple electrical circuit having battery and galvanometer.
At first, the length (L1) of the sample is measured. The set up is shown in the figure below, where the sample is kept inside the hollow cylinder, thermometer and heat supply kit are connected as shown in the figure. Above the hollow cylinder, spherometer is adjusted in such a way that middle leg of it touches the sample and other three legs are placed in an insulating stand. To check the connectivity of the middle leg of spherometer with a sample an electrical circuit is introduced.

 If galvanometer shows deflection it ensures the connectivity and the initial readings of the thermometer (θ1) and speedometer reading (R1) are taken.
After that screw of spherometer is slightly rotated up so that there exists a gap for expansion, in this condition galvanometer, doesn’t show the deflection. Now steam is passed through the kit continuously until the thermometer shows a steady temperature. Again the screw is rotated down so that it touches the sample and deflection can be observed in the galvanometer. For this condition, steady temperature (θ2) and final spherometer reading ((R2) are taken.
 Now we have a change in length i.e R2- R1.
Change in temperature is θ2 - θ1
Initial length is L1, so putting all values in the formula of change in length.

Where alpha is the coefficient of linear expansion.
In this way, the coefficient of linear expansion of the rod is determined.

 Yes, cubical expansivity can be determined from this value because,   
Cubical expansivity =3 times linear expansivity.

Some previous year questions and solutions:




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