Given, 9 divisions of main scale are equal to 10 divisions of Vernier scale.
i.e. 9 MSD = 10 VSD
⇒VSD=‌MSD ...(i)
Size of 1 main scale division,
1 MSD = 1 mm
Now, least count, LC = 1MSD − 1VSD ...(ii)
Using Eqs. (i) and (ii), we get
LC=1‌MSD−‌MSD=‌MSD=‌mm While measuring the diameter of bob.
Main Scale Reading, MSR = 10 mm
Vernier Scale Reading, VSR = 8
Zero error, e = 0.04 cm
Now, diameter,
d=[MSR+LC×VSR]−e =(10‌mm+×8‌mm)−0.04‌cm =(10.8)‌mm−0.04‌cm =1.08‌cm−0.04‌cm=1.04‌cm Radius,
r==‌cm=0.52‌cm=52×10−2‌cm ∴ Correct answer is 52.