In an LVDT, when the core is at NULL position, the flux linkage with both the secondary windings is __ and results in output voltage.

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HPCL Engineer Instrumentation 11 Aug 2021 Official Paper

Answer 1

The term LVDT stands for the Linear Variable Differential Transformer. It is the most widely used inductive transducer that converts the linear motion into the electrical signal.
The output across secondary of this transformer is the differential thus it is called so. It is very accurate inductive transducer as compared to other inductive transducers.
Continuous alternating current is applied to the primary coil, generating what we call primary excitation and a varying magnetic field.
This primary excitation interacts with the secondary coils, which then induces current and voltage to be produced in the secondary coils.
When the core is at the null position then the induced magnetic flux to each of the secondary coils is equal.
We know that they are electrically 180 degrees out of phase with each other, therefore, they will cancel each other, and the output voltage will be zero. This characteristic is used when calibrating an LVDT sensor.