Scope soldering iron
The soldering iron's "element" is a small carbon tip at the end of a pushrod connected to one terminal of the power supply, inside the stainless-steel barrel of the tool, but insulated from it. The barrel and its copper tip (the "bit"), is connected to the other terminal. When the user operates the pushrod, the carbon element bears against the back of the "bit", resulting in electric heating of the resistive carbon. The user modulates the temperature of the "bit" by the pressure they exert on the pushrod, through a lever or sliding ring, depending on the model.