The evaporator (1) has a wick (4) projecting from a container (2), and a cap (3) provided for covering the wick. The cap has a guide (11) that permits covering of wicks of different lengths using the cap. The guide allows lateral movement between the container and the cap. A thread (12) is provided in the guide. The container has a reservoir area (14), where the diameter of the guide is lesser than the diameter of the reservoir area. The outer diameter (17) of the cap corresponds to the diameter (15) of the container. The cap has a wick cover (21) lying radially within the guide.