Question

in an experiment to find the young modulus , the strain should not be more than 1 in 1000 , explain this statement

Answer 1

Young's modulus is a ratio for metal - metal wire, metal rod, metal beam etc.
Y = stress/strain {in the ELASTIC region of materials ie metals}

Strain = (Lf - Lo)/Lo,

where Lf is final length and Lo is initial length.
If
strain < 0.001
then
(Lf - Lo)/Lo < 0.001

and from that

Lf - Lo < 0.001(Lo).

Keeping the "change in length" below 0.1% is necessary to prevent the "elastic limit"
of the metal from being exceeded. IF a metal's elastic limit is exceeded "Young's modulus" no longer = stress/strain so any experiment to find its value that went beyond the elastic range would be void.