Question

A U-tube contains water and methylated spirit separated by mercury. The mercury columns in the two arms are in level with 10.0 cm of water in one arm and 12.5 cm of spirit in the other. What is the specific gravity of spirit ?

Answer 1

The given system of water, mercury, and methylated spirit is shown as follows :

Height of the spirit column, h1 = 12.5 cm = 0.125m
Height of the water column, h2 = 10 cm = 0.1 m
P0 = Atmospheric pressure
ρ1 = Density of spirit
ρ2 = Density of water
Pressure at point B = P0 + h1ρ1g
Pressure at point D = P0 + h2ρ2g
Pressure at points B and D is the same.
P0 + h1ρ1g = h2ρ2g

$= \frac{ρ_1 }{ ρ_2} = \frac{h_2 }{ h_1}$

$=\frac{ 10 }{ 12.5 }= 0.8$

Therefore, the specific gravity of spirit is 0.8.