Correct Answer - Option 4 : Electric resistance strain gauge method
Concept:
Electric strain Gauge method is mostly used by Indian Railway.
This method is based on the principal that any change in any dimension of element under consideration, its resistance against flow of current changes in that direction. The electrical resistance strain gauge is a resistance element which changes resistance when subject to strain. A rail section is strained to certain extent due to which its length either decreased or increased. This change in length leads to change in its resistance. This resistance is measured, then used to calculate the stresses in rail.
