Q.Why is a low density liquid used as a manometric liquid in a physics laboratory?

Two standard instruments measure pressure by balancing it against the weight of a standing liquid column. A mercury barometer measures atmospheric pressure directly: a glass tube completely filled with mercury is inverted into an open dish of mercury, and the mercury remains standing at a height h inside the tube, supported against a near-perfect vacuum (Torricelli's vacuum) at the tube's sealed top; since the pressure at the top of this column is essentially zero, and the pressure at the same level as the open dish's surface is atmospheric pressure $p_0$, the relation $p_0 = h\rho g$ (ρ being mercury's density) lets atmospheric pressure be read straight off the column's height — the standard value being 76 cm of mercury. …