The Wetted Bulb Apparatus is a self-made construction which can simultaneously measure mass and heat transfer. This is realized by the evaporation of water through a ceramic bulb ("wetted bulb") in air. The water supply occurs by a capillary which contains a temperature sensor. It is possible to condition the air with a humidity between 0 and 100% RH. The Wetted Bulb Apparatus is used by students in practical exercises to study the theory of mass and heat transfer using the numbers of Nusselt, Sherwood, Schmidt and Reynolds.