Permanent adjustment of Plate Level Axes of transit |
The axis of each plate level should lie in a plane perpendicular to the vertical axis. Test : In order to test it, one of the level tubes is brought parallel to any two of the foot screws, by rotating the upper part of the instrument (by clamping the lower plate main screw and opening the upper plate main screw). The bubble is brought to the centre of the level tube by rotating foot screws. The level tube is then brought over the third foot screw again by rotating the upper part of the instrument. The bubble is then again brought to the centre of the level tube by rotating the third foot screw. These operations are repeated by rotating the upper part of the instrument in the same quadrant and levelling till the bubble remains central in both the positions. Now clamp the upper plate main screw and open the lower plate main screw and rotating the whole instrument through 180°. If the bubble in the plate level remains in the centre, the axis of the plate level lie in a plane perpendicular to the vertical axis. Similar, test has to be carried out for the other level tube also. Adjustment : If the bubble in any level tube gets displaced from centre, bring it back halfway by adjusting two foot screws parallel to it (Figure 21.3) and other half by adjusting the capstan screws fitted at one end of the plate level tube. Rotate the whole instrument through 180° and repeat the steps of testing and adjustment if required till the bubble remains central for all positions of the instrument as it revolved through 360° about the vertical axis.
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