Without any object in between the spindle and the anvil, fully close them by rotating the thimble head. If the circular scale zero mark has gone beyond the main scale zero, then there is a negative zero error.

If the pitch of the screw gauge is 0.5 mm, the main scale has been divided to read up to 0.5 mm and the circular scale has 50 subdivisions, then the least count of the instrument is 0.01 mm.

The number of circular divisions that must be rotated back to make the circular zero mark in line with the base line of the main scale gives the negative zero error.

Zero error = -(number of circular divisions rotated back x 0.01 mm)

E.g.: If circular mark 36 is in line with the main scale base line, then the number of divisions that must be rotated back is (50-36) =14.

Zero error= -0.14 mm.

Remember that the zero error (whether it is positive or negative) is always deducted from the scale reading.

Therefore, positive error decreases the value and negative error increases the value.

Dr. Sarath Bandaranayake, B.Sc., Ph.D.

If the pitch of the screw gauge is 0.5 mm, the main scale has been divided to read up to 0.5 mm and the circular scale has 50 subdivisions, then the least count of the instrument is 0.01 mm.

The number of circular divisions that must be rotated back to make the circular zero mark in line with the base line of the main scale gives the negative zero error.

Zero error = -(number of circular divisions rotated back x 0.01 mm)

E.g.: If circular mark 36 is in line with the main scale base line, then the number of divisions that must be rotated back is (50-36) =14.

Zero error= -0.14 mm.

Remember that the zero error (whether it is positive or negative) is always deducted from the scale reading.

Therefore, positive error decreases the value and negative error increases the value.

Dr. Sarath Bandaranayake, B.Sc., Ph.D.