A unit scale is to be prepared whose length does not change with temperature and remains 20cm using a bimetallic strip made of brass and iron each of different length. The length of both components would change in such a way that difference between their lengths remains constant. If length of brass is 40cm and length of iron will be ––––––––cm.
$\left({\mathrm{\alpha }}_{\text{iron }}=1.2\times {10}^{-5}{\mathrm{K}}^{-1}\text{ and }{\mathrm{\alpha }}_{\text{brass }}=1.8\times {10}^{-5}{\mathrm{K}}^{-1}\right)$