A length of 2 m is measured using a metal tape at 10^oC. It is calibrated at a temperature 30^oC. The actual length is ($\mathrm{\alpha }= 1\times {10}^{-4}{/}^{\mathrm{o}}\mathrm{C}$)

Error = $∆\mathrm{l}=\mathrm{\alpha l}\left({\mathrm{t}}_2-{\mathrm{t}}_1\right)={10}^{-4}\left(2\right)(30-10)$ = 0.0004

Actual length =measured length - error ; when metal tape is used at lower temperature.  
  $\therefore$  Actual length = 2 – (0.004) = 1.996 m