A vacuum tube needs an extremely good ("hard") vacuum to avoid the consequences of generating positive ions within the tube. With a small amount of residual gas, some of those atoms may ionize when struck by an electron and create fields that adversely affect the tube characteristics. Larger amounts of residual gas can create a self-sustaining visible glow discharge between the tube elements. To avoid these effects, the residual pressure within the tube must be low enough that the mean free path of an electron is much longer than the size of the tube (so an electron is unlikely to strike a residual atom and very few ionized atoms will be present). Commercial vacuum tubes are evacuated at manufacture to about .