Filling glass tube with mercury
Take a thick-walled glass tube with one end closed and fill it with mercury. Try filling it to the brim such that there are least chances of having any air trapped in the tube.
Filling a dish with mercury
Now take a small dish made up of glass and fill it with mercury. While you are filling it with mercury make sure that you don’t let t over flow as mercury is quite an expensive item.
Create Vacuum above mercury
Now pick up the glass tube that you earlier filled with mercury. In order to ensure that there are no bubbles of air inside the tube fill more mercury with help of a dropper. You may also connect a vacuum pump for drawing mercury into the tube and keeping a vacuum at the top of the barometer.
Inverting Glass Tube on Mercury Dish
Place your finger at the top of the glass tube and invert it over the open mercury dish. After that, slowly remove your finger from the open end of the glass tube.
As you will remove the finger, it will be observed that the level of mercury in the glass tube begins to fall. However, at one point it will stop when there will be atmospheric pressure acting on the surface of mercury in dish.