The instrument used is the Vernier Caliper.
It is a precision measuring tool that helps measure external dimensions, internal dimensions, and depth with high accuracy.
In the adjoining figure, $\triangle CAB$ is a right triangle, right angled at A and $AD \perp BC$. Prove that $\triangle ADB \sim \triangle CDA$. Further, if $BC = 10$ cm and $CD = 2$ cm, find the length of AD. 