External Immersion Heater

immersion heater
An electric heating element that is positioned in the liquid to be heated
Electric heating is any process in which electrical energy is converted to heat. Common applications include heating of buildings, cooking, and industrial processes.
a heating element that is immersed in the liquid that is to be heated (as in a hot-water tank)
a heating device used to maintain a constant temperature in the mash tun.

external
happening or arising or located outside or beyond some limits or especially surface; "the external auditory canal"; "external pressures"
outward features; "he enjoyed the solemn externals of religion"
Relating to or denoting a medicine or similar substance for use on the outside of the body
coming from the outside; "extraneous light in the camera spoiled the photograph"; "relying upon an extraneous income"; "disdaining outside pressure groups"
Belonging to or forming the outer surface or structure of something
Coming or derived from a source outside the subject affected