The heat transfer constitutes an important physical unit for resistor design. The heat transfer coefficient a indicates how much power in Watt [W] per surface area unit A= cm², m², and at a temperature increase of 1 Kelvin [K] can be transferred to the surrounding medium. a depends on the structure of the surface (smooth, rough), the cooling medium (gaseous, liquid, solid) and the ambient temperature. Particularly gaseous coolants such as air do not lend themselves to an exact measurement of the heat transfer because it varies considerably as a function of the air temperature and the flow speed resulting from natural or forced circulation. Turbulences due to rough or structured surfaces improve the heat transfer because they prevent the formation of layers. A heat transfer coefficient W/(cm² K) means that the surface temperature of a 200.0 = a resistor under a load of 0.6 W/cm² will rise by 0.6/0.002 = 300 K.