A glowplug (alternatively spelled as glow plug or glow-plug) is a heating device used to aid starting diesel engines. The glow plug is a pencil-shaped piece of metal with a heating element at the tip. This heating element, when electrified, heats due to its electrical resistance and begins to emit light in the visible spectrum, hence the term "glow" plug. The effect is very similar to that of a toaster. Heat generated by the glowplugs is directed into the cylinders, and serves to warm the engine block immediately surrounding the cylinders. This aids in reducing the amount of thermal diffusion which will occur when the engine attempts to start.