The diameter of a barometer tube is 3 mm. What will be the error in the barometer reading due to surface tension? For mercury in the glass tube, the surface tension `=0.647 N.m^-1`, the angle of contact `=128^@` and the density of mercury `=13500 kg. m^-3`.

Explanation

We know that `h=(2 T cos theta)/( r rho g)`
Here, `T=0.647N. M^-1, theta=128^@`,
`r=(3xx10^-3)/(2)=1.5xx10^-3m,`
`rho=13500 kg. m^-3 , g=9.8m . S^-2`,
`cos theta=cos 128^@=cos(90^@+38^@)`
`=-sin38^@=-0.6157`
`therefore h=-(2xx0.647xx0.6157)/(1.5xx10^-3xx13500xx9.8)`
`=-0.004`m
So, the obtain the actual atmospheric pressure, 0.004 m has to be added to the barometric reading.