In instrument like a voltmeter, because of the small distance between the scale and point of needle directly above the scale, the needle appears to point to different points on the scale when viewed from different directions. For taking accurate readings the position of needle must be observed from a point which is vertically above the position of the needle.
A mirror mounted under the needle and near the scales of an helps take more accurate readings by enabling the observer to locate the point on the scale that is directly under the needle showing the reading. To do this this the observer looks at the voltmeter from a position at which the needle completely hides the image of the needle in the mirror. The observer then notes down the point on the scale that the needle covers points to from this direction of viewing the scale.