English: A steel mill or device for illuminating coal mines before safety lamps were habitual. Toothed gearing spun a steel wheel D against which a flint was held. The sparks were sufficient to light the work, and were too small to ignite any explosive gas - usually. From the change in colour of the sparks an experienced operator could tell if the atmosphere was getting dangerous. It was not foolproof, but much safer than the previous form of illumination: lighted candles. With this tool, areas of mines that had been considered too dangerous to work became exploitable.