Question

To construct a barometer, a tube of length $1m$ filled completely with mercury and is inverted in a mercury cup. The barometer reading on a particular day is $76cm$. Suppose a $1m$ tube is filled with mercury up to $76cm$ and then closed by a cork. It is inverted in a mercury column in the tube over the surface in the cup will be

• A. Zero
• B. $76cm$
• C. $>76cm$
• D. $<76cm.$

Answer 1

The correct option is D $<76cm.$

The tube contains air $($because it is not fully filled$).$ This air pressure against atmosphere pressure$,$

therefore$,$ height of column $<76cm$

Hence,

option $\left(D\right)$ is correct answer.