Although there are various kinds of mug warmers made by different manufacturers, they're usually shaped like the drink coasters people use on tabletops to prevent water rings from damaging the surface.
Mug warmers resemble drink coasters, but they're thicker and generally have a slightly greater circumference than most drink coasters. They're typically designed for mugs, but you don't have to limit their use to just drinks. Other hot foods, like soups and stews, can stay warm if you pour them into a bowl and place the bowl on top of the device. Many designs have electric cords of varying lengths so you can plug the warmer into an outlet to receive power, but some models have a USB cable that lets you plug directly into a desktop or laptop computer for power, similar to the way you can charge an MP3 player with a USB port.
Mug warmers use heating coils in the same way electric kettles do. Once plugged in and switched on (most have an on/off switch on the side), electricity flows to a small heating coil built inside the device. A mug warmer works by using the simple properties of heat transfer and thermal energy. If we look at temperature on a microscopic scale, a material with a higher thermal energy just means the molecules in that material have a high kinetic energy and are moving around much faster than molecules in a cooler material. The molecules in a hot cup of coffee, for instance, are moving faster than the molecules in a cold cup of coffee. As the molecules collide, some of that energy transfers, warming up the cooler molecules and cooling down the hotter molecules in a process known as conduction. When you put your cool hands against a warm cup of coffee or tea, your hands will eventually get warmer because heat is transferring from the cup to your skin. So the mug warmer's coils transfer heat to the base of the mug warmer, and once you place a cup onto the device, the heat from the base will transfer to the cup and the liquid within.
So for the forgetful and the often overwhelmed, this handy device can keep coffee, tea, hot chocolate or soup at a constant, warm temperature without heating it beyond the boiling point.
For more information on other food conveniences, warm up to the links on the next page.