Zero Errors of a Screw Gauge



Zero errors of a screw gauge:
    Some times the zeroth division of the head scale does not coincide with the index line but lies below or above the index line, when S1 and S2 are in contact. This happens due to a fault in the instrument while manufacturing it or by the frequent use of the instrument. This error is known as zero error.

Suppose 2nd division of the head scale coincides with the index line when S1 and S2 are in contact. The zeroth division of the head scale lies 2 divisions below the index line. So the observed head scale reading (HSR) will be 2 divisions more than the correct reading. This error is known as positive zero error. To get correct HSR, 2 divisions should be subtracted from the observed HSR so the zero correction is negative.

     Suppose 98th division of a head scale coincides with the index line when there are 100 divisions on the head scale. The zeroth division of the head scale lies 2 divisions above the index line. The observed HSR will be 2 divisions less than the correct reading. So this error is known as negative zero error. To get correct HSR, 2 divisions should be added to the observed HSR. So the zero correction is positive.

To derive a formula for the thickness of a glass plate:
    When the given object is fixed between S1 and S2, the screw moves backward, a distance equal to the thickness of the object. This distance is noted from the pitch scale. If the edge of the head scale coincides with a division on the pitch scale and zeroth division of head scale coincides with the index line, then that reading can be taken as the thickness of the object, when there is no zero error. Some times the edge of the head scale lies in between two divisions of the pitch scale. The division that just precedes the edge is taken as pitch scale reading (PSR). The thickness of the object will be equal to PSR + distance between PSR and the edge. The division of the head scale that coincides with the index line gives the head scale reading (HSR). We know that the rotation of one division of head scale makes the screw to move a distance equal to the least count. So the product of HSR and L.C. gives the distance between P.S.R. and the edge.

                    Thickness = P.S.R. + H.S.R. × L.C.

    If there is zero error, then thickness = P.S.R.+C.H.S.R. × L.C., where C.H.S.R. is the corrected head scale reading.

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