= Mercury-barometer temperature correction
{title2=$h_s=\rho(T)g h/(\rho_0g_s)$}
A measured mercury column represents <pressure> through its actual <mass density> and local <gravitational acceleration>. An equivalent reference column therefore obeys $\rho_0g_sh_s=\rho(T)gh$. If the indicated scale length expands as $1+\alpha_s\Delta T$ and mercury volume as $1+\beta\Delta T$, the first-order thermal factor multiplying the scale reading is $1+(\alpha_s-\beta)\Delta T$. These factors describe a pressure-unit reduction, not a physical change in local <gravity>.<Mercury Barometers and Manometers by W. G. Brombacher and others>[§§9–10 and equation45]{ref}
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