A linear variable differential transformer (LVDT) is a type of electrical transformer used for measuring linear displacement.
The transformer has three solenoidal coils placed end-to-end around a tube. 
The centre coil is the primary, and the two outer coils are the secondaries. A cylindrical ferromagnetic core, attached to the object whose position is to be measured, slides along the axis of the tube. As the core moves, these mutual inductances change, causing the voltages induced in the secondaries to change which can be easily measured.
Applications
- Using Spring loaded LVDT for displacement measurement of imbedded concrete anchors tested for tensile, compression, bending strength and crack growth in concrete.
Spring loaded LVDTs are used for measuring deformation and creep of concrete wall used for retaining wall in large gas pipe installation.- Free core LVDTs used in dynamic measurement of fatigue in large structural components used in suspension bridges.
- Spring loaded LVDT used in down-hole application: measuring displacement (creep) of bedrock.