A Barometer kept in an elevator reads 760 mm when the elevator is at rest. When the elevator moves in the upward direction with increasing speed, the reading of the Barometer will be :

A mercury barometer measures atmospheric pressure (P) using the relation P = ρgh, where ρ is the density of mercury, g is the acceleration due to gravity, and h is the height of the mercury column. When an elevator accelerates upward with acceleration 'a', it becomes a non-inertial frame. The effective gravitational force (g_eff) inside the elevator increases to (g + a). Since the external atmospheric pressure (P) remains constant (assuming the elevator is not airtight or the change in altitude is negligible during the acceleration phase), the equilibrium equation becomes P = ρ(g + a)h'. Because the effective gravity (g + a) is greater than the standard gravity (g), the height of the mercury column (h') must decrease to maintain the same pressure balance. Consequently, the barometer reading will be less than 760 mm.