What is an extrometer, how many types are available?

Question

It is a measuring instrument.

Answer 1

*An Extrometer is designed to record simultaneously the extension and rotation over a predetermined gauge length.

Strain gauges are used to measure the surface strains on the wires in the outer layer of the strand.

Preliminary tests on seven-wire strands demonstrate that the extrometer instrument provides reliable results. The extension and rotation characteristics recorded on a seven-wire strand under tensile load agree reasonably well with the corresponding theoretical predictions, but the wire surface strains reveal an unequal load sharing between nominally identical helical wires.