Oh, heck, it's not that complicated.
Power source, heating element, juice jug.
Of course, there are a lot of power sources, types, configurations, VV, VW, PWM, RMS, and car batteries....
Of course, there area lot of heating elements, resistance, Kanthal, Nichrome, wick, wickless, silica, cotton, bamboooooooooo....
Of course, there are a lot of juice jugs, cartos, clearos in 87 styles, RBA, QRS, TUV, and whatever else I feel like making up
Nothing to it
