Question

Why is it customary to use some lead shots or mercury in the lower bulb of a hydrometer?

Answer 1

This is done so as to lower the center of gravity of the hydrometer so that it remains in a stable equilibrium and floats in an upright position.

Hope this helps:)

Answer 2

the lower bulb of a hydrometer is filled with lead shots or mercury to make the hydrometer float vertically in liquids.