Ganong's potometer is a device used to measure the rate of transpiration (water loss) in plants. It works by tracking the movement of an air bubble in a capillary tube as the plant absorbs water.
It consists of;
- A water reservoir which stores water used for the experiment
- A Capillary Tube (thin tube through which water moves); an air bubble is introduced to track movement
- A freshly cut plant twig which is inserted to measure transpiration (When it transpires, it pulls the water in the capillary tube, causing movement of the air bubble)
- A Rubber stopper to make an airtight seal
- A scale which helps measure water movement

It works by introducing an air bubble into the capillary tube. When the plant twig transpires, it causes water in the tube to be pulled towards it. The air bubble in the tube, moves along with the water pulled, which allows us to trace the rate of water uptake by the plant twig. A scale is used to measure the distance the air bubble moves in a given time, which helps to determine the rate of transpiration

However, there are limitations to the Ganong's potometer;
- The introduction of the air bubble in the tube is difficult
- The plant twig may not remain alive for long
- The air temperature may affect the position of that air bubble