The one division of main scale of vernier calipers reads 1mm and 10 division of Vernier scale is equal to the 9 divisions on main scale. When the two jaws of the instrument tough each other the zero of the Vernier lies to the right of zero of the main scale and its fourth division coincides with a main scale division. When a spherical bob is tightly placed between the two jaws, the zero of the Vernier scale lies in between 4.1 cm and 4.2 cm and $6^{\text {th }}$ Vernier division coincides with a main scale division. The diameter of the bob will be $\_\_\_\_$ $10^{-2} \mathrm{~cm}$.