Question

When we observe the level of mercury in a capillary we note down the level of the tangent to the meniscus above the mercury column.

Answer 1

Mercury forms a convex meniscus in a capillary tube because its cohesive force is greater than its adhesive force with glass. Therefore, while measuring the mercury level, we take the reading at the highest point of the convex meniscus, i.e. along the tangent drawn above the mercury column.