Answer: Linear Variable Differential Transducer( LVDT) - It is a variable inductance displacement transducer.

 Construction - It consists of a primary and two identical secondary winding. These windings are axially spaced and wound on a cylindrical core former. A rod shaped core is positioned centrally inside the coil assembly. This rod provides a low reluctance path for the magnetic flux linking the coils. The displacement pf which is to be measured is coupled to this movable rod.

Since , the two secondary winding are connected in series so the voltages induced into winding are of opposite polarities. The output voltage is given as 

$e_0=e_{01}-e_{02}$

where, $e_{01} \ and \ e_{02}$ are the emf induced in two secondary windings.

Operation - The primary winding is connected to the ac source. According to the position of the core , three cases are developed which are given below.

Case 1 : When the core is exactly in center of the coil assembly.Then , the flux linked to both the coils of secondary are equal. Due to this the secondary induced voltages are equal but with opposite polarities.The output voltage is zero and this position is called as Null position.

Case 2 : When the core is displaced towards secondary 1 , then the flux linkage to secondary 1 increases and flux linkage to secondary 2 decreases. Therefore, $e_{01}$ is greater than $e_{02}$ . Hence, the output voltage is positive.

Case3 : When the core is displaced towards secondary 2 , then the flux linkage to secondary 2 increases and flux linkage to secondary 1 decreases. Therefore, $e_{02}$  is greater than $e_{01}$  Hence, the output voltage is negative.

Thus , a voltage proportional to the displacement of core is obtained at the LVDT output. Hence , LVDT helps to measure the displacement of the moving object.