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A Glow Plug (alternatively spelled as glowplug or glow-plug) is aheating device used to aid starting dieselengines. The glow plug is a pencil-shaped piece ofmetal with a heatingelement at the tip. This heating element, when electrified, heatsdue to its electricalresistance and begins to emit light in the visible spectrum, hence theterm "glow" plug. The effect is very similar to that of a toaster. Heat generated by the glow plugs is directed into thecylinders, and serves to warm the engine block immediately surrounding thecylinders. This aids in reducing the amount of thermal diffusion which will occur when the engine attempts to start.