A unit scale is to be prepared whose length does not change with temperature and remains \( 20 \,\text{cm} \), 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 \( 40 \,\text{cm} \) and length of iron will be ______ \( \text{cm} \).
(\( \alpha_{\text{iron}} = 1.2 \times 10^{-5} \,\text{K}^{-1} \) & \( \alpha_{\text{brass}} = 1.8 \times 10^{-5} \,\text{K}^{-1} \)).
