A student measured the diameter of a small steel ball using a screw gauge of least count 0.001cm. The main scale reading is 5mm and zero of circular scale division coincides with 25 divisions above the reference level. If screw gauge has a zero error of -0.004cm, the correct diameter of the ball is?

Least count = 0.001cm
Main Scale Reading (MSR) = 5mm = 0.5cm
Circular Scale Reading (CSR) = 25 divisions
Zero error = -0.004cm

Observed reading = MSR + CSR × Least count
Observed reading = 0.5cm + 25 × 0.001cm = 0.5cm + 0.025cm = 0.525cm

Actual reading = Observed reading + Zero error
Actual reading = 0.525cm + (-0.004cm) = 0.521cm

Therefore, the correct diameter of the ball is 0.521cm