To measure the rate of the transpiration current.
A potometer is filled with water, and a branch cut under water is fixed air-tight to the upper wide end of the apparatus through a cork. The distal end of the apparatus is dipped in a beaker containing water which may be coloured with eosin.
As transpiration occurs, the coloured water is seen to enter the tube. Remove the end of the tube from the beaker for a while and allow air to enter it. Dip it into water again. The air-bubble is seen to travel through the horizontal arm due to suction exerted by transpiration. Note the time that the bubble takes to cover the journey from one end of graduation to the other.
The volume of the graduated tube being known, the rate of transpiration current is easily determined. By opening the stopcock the bubble may be pushed back and the experiment re-started.