SCREW GAUGE

If on bringing the flat end of the screw in contact with the stud , the zero mark of the circular scale coincides with the zero mark on base line of the main scale , the instrument is said to be free from zero error. But if it is not so, the instrument is said to have a zero error.

The zero error is of the following two kinds :-

(i) Positive zero error .

( ii) Negative zero error.

To find the positive zero error, we note the division of the circular scale which coincides with the base line. This no. when multiplied with the least count of the screw gives the positive zero error.

To find the negative zero error , we look for the division of the circular scale coinciding with the base line. This no. is subtracted from the total no. of divisions on the circular scale and is then multiplied with the least count of the screw guage . This gives the negative zero error.