Boiling-water canners are made of aluminum or porcelain-covered steel. They have removable perforated racks and fitted lids. The canner must be deep enough so that at least an inch of briskly boiling water will be over the tops of jars during processing. Either a flat or ridged-bottom canner can be used on a gas burner, but a flat bottom must be used on an electric range. To ensure uniform processing of all jars with an electric range, the canner should be no more than four inches wider in diameter than the element on which it is heated.
Steam Pressure Canner
Modern pressure canners are lightweight, thin-walled kettles. They have a jar rack, gasket, dial or weighted gauge, an automatic vent/cover lock, a vent port (steam vent) to be closed with a counterweight or weighted gauge, and a safety fuse.
Following strict canning guidelines, like the ones detailed here, is important. If done improperly, growth of the bacterium Clostridium botulinum in canned food may cause botulism—a deadly form of food poisoning. For more information on proper canning procedures, visit The National Center for Home Food Preservation website at nchfp.uga.edu.
Source: The National Center for Home Food Preservation