501 Auto Distiller Heating Element. This is a rod style heating element that is attatched with 1 single large brass nut.
The 501 heating elements can be either 900 watts or 1000 depending on when your distiller was manufactured.
The most common reasons for this element to fail is too much mineral build-up in the boil chamber, or if the user drains the boil chamber while the unit is still operating or was just operating. In both cases this could cause the element to overheat or shorten its life.