A cup heater coil is a small electric appliance designed for heating water inside a cup or mug. Also called an immersion heater, the device consists of a heating coil that is inserted in the water and a small cord that plugs into a wall outlet.
These devices are designed for travel and other locations, such as workplaces, where other methods of heating water might not be available. Because of this, many of the devices come equipped with adapters for use on both 120- and 240-volt electrical power systems. The heaters are relatively small and efficient, and they quickly heat a cup of water to the desired temperature.