Rate of transpiration is measured by

The rate of transpiration is measured by Ganong's potometer.
A potometer, is a device used for measuring the rate of water uptake of a leafy shoot which is almost equal to the water lost through transpiration.
Francis DarwinCharles Darwin Ganong Bose designed and invented Ganong’s potometer. The Ganong's potometer is composed of a glass tube, which is bent twice, and a glass cylinder with a wide mouth also a capillary tube is inserted into a horizontal glass bar attached to a reservoir.