Up to now there has not been a device with which it is possible to heat or control the temperature in particular of small amounts of liquids that are present in any non-metallic vessel without using a cable-connected heating device for this purpose. A method and devices have been developed with which it is possible, for example, to heat a drink in a paper cup to the exact degree and keep it at the preselected temperature without having to heat the vessel or inserting a heating device inton the liquid. For this purpose, a method has been developed with which a heating element, which is inserted into a non-metallic vessel filled with a liquid, is inductively heated, whereby the temperature of the liquid in the vessel and the surface temperature of the heating element are determined at the same time, and thus individually adaptable heating and temperature control in any type of non-metallic vessel can take place. The input of thermal energy is very energy-efficient, since only the medium in which the heating element is located is heated and thus takes place by direct input of energy. In this way, for example, such a device can be installed and operated in a motor vehicle, as a result of which, for example, coffee in a disposable cup can be kept at a selectable temperature over the entire duration of consumption.