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?
Figure showing heights of liquids in an U-Tube
Given:
Height of spirit column (h1) = 12.5 cm
Height of water column (h2) = 10.0 cm
Let P0 = Atmospheric pressure
ρ1 = density of the spirit
ρ2 = density of the water column
Pressure due to spirit column at B = P0 + ρ1gh1
Pressure due to water column at D = P0 + ρ2gh2
Pressure is equal at the points having the same height from the ground i.e. at points B and D.
P0 + ρ1gh1 = P0+ ρ2gh2
ρ1h1 = ρ2h2
We know that 
Therefore specific gravity of spirit is 0.8.
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