In a screw gauge when the circular scale is given five complete rotations it moves linearly by \( 2.5 \, \text{mm} \). If the circular scale has \( 100 \) divisions, the least count of screw gauge is ______ \( \text{mm} \).
\( 1 \times 10^{-2} \)
\( 1 \times 10^{-3} \)
\( 5 \times 10^{-2} \)
\( 5 \times 10^{-3} \)