Teardown of a thermally controlled soldering iron element.

This is what's inside a typical 24V 50W soldering iron heating element. I actually thought it was going to be simpler than this, but it's quite impressively engineered.
The construction is basically a ceramic core and outer ceramic sleeve. The core has three small holes up its full length and these are used to insulate the wires for the thermal sensor and heating element from each other. The thermal sensor is a common thermocouple composed of two dissimilar metals fused together at the tip of the iron. These generate a small voltage that relates directly to the tip temperature. I estimate that at a typical iron temperature of 350 degrees the sensor will produce a voltage of about 14mV which is monitored by an op-amp in the controller.
The heating element itself is wound round a short section of the inner ceramic core near the tip and terminated by simply running the same resistance wire to the connection PCB in the soldering irons handle, but also running it with a compatible but much lower resistance wire that is twisted along the full length that isn't intended to heat up. This seems to be a common way to terminate heater wires, since it provides a low stress connection.
The inner core with heating element and thermocouple are then slid into an outer ceramic tube and sealed at both ends with a ceramic paste. The protruding wires are sleeved with a high temperature sleeve and the ceramic paste is also used to glue them in position.
Although the irons element is rated at 24V the transformer winding that feeds it is rated over 30V to allow for losses over the opto-isolated triac that switches it and the resistance of the thin cable to the iron itself.