
Concept explainers
Left thumb rule in levelling.

Explanation of Solution
Left thumb rule is applicable in leveling for the measurement of vertical angles. In order to use the instruments effectively we must follow some of the basic principles.
Most of the latest instruments use three screw leveling head for initial rough leveling. In leveling a three-screw head, the telescope is rotated until it is over two screws that is, horizontal to the two screws, and then by using the thumb and the first finger of each hand in order to adjust the opposite screws simultaneously fixing the bubble approximately at the center (if the left thumb is turned to the left as moving right thumb to right it will turn the bubble towards left, similarly moving left thumb to right and right thumb to left will make the bubble to move towards right). The whole procedure is repeated with the telescope at 90oso that it is over the third remaining screw just opposite the other two. Now, a lot of time gets wasted while adjusting the bubble with the first attempt.A simple rule that assists in centering bubble, as a bubble follows the left thumb rule when turning the screws gets centered by turning the screws one after the other. Furthermore, the inward and outward movement of the foot screws will easily center the instrument and telescope need not to be rotated throughout the process.
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