A mercury barometer is placed in an elevator accelerating upwards with a constant acceleration (assume constant temperature and no air leaks). If h is the height of mercury column in the stationary elevator and H is the height of mercury column in the moving elevator, then which one of the following is correct?

A mercury barometer measures atmospheric pressure (Patm) based on the equilibrium between the weight of the mercury column and the external air pressure. The relationship is given by Patm = ρgeffH, where ρ is the density of mercury and geff is the effective acceleration due to gravity. In a stationary elevator, geff equals the standard acceleration due to gravity (g), so Patm = ρgh. When the elevator accelerates upwards with acceleration 'a', an additional downward pseudo-force acts on the mercury, making the effective gravity geff = g + a. Assuming the atmospheric pressure (Patm) remains constant, the balance becomes Patm = ρ(g + a)H. Comparing the two cases: ρgh = ρ(g + a)H. This leads to H = h × g/(g + a). Since (g + a) > g, it follows that H < h.