10.10 In the previous problem, if 15.0 cm of water and spirit each is further poured into the respective arms of the tube, what is the difference in the levels of mercury in the two arms? (Specific gravity of mercury = 13.6)

Height of the water column, h1 = 10 + 15 = 25 cm

Height of the spirit column, h2 = 12.5 + 15 = 27.5 cm

Density of water, ρ1 = 1 g cm–3

Density of spirit, ρ2 = 0.8 g cm–3

Density of mercury = 13.6 g cm–3

Let h be the difference between the levels of mercury in the two arms.

Pressure exerted by height h, of the mercury column:

= hρg = h × 13.6g                       ………. (i)

Difference between the pressures exerted by water and spirit:

=h1ρ1g-h2ρ2g

= g(25 × 1 – 27.5 × 0.8) = 3g       ……… (ii)

Equating equations (i) and (ii), we get:

13.6 hg = 3g 

h =313.6

=0.220588 ≈ 0.221 cm

Hence, the difference between the levels of mercury in the two arms is 0.221 cm.