The metal cap that is firmly secured to the back of the soldering tip is the temperature sensor of this system; the Magnastat. It is ferromagnetic, i.e. it attracts iron as does a magnet. The number on the cap is the code for the control temperature, when the iron is heated. When this control temperature is reached, the Magnastat loses its magnetic property. Physicists call this the Curie effect. Once the temperature falls below the control temperature, the Magnastat regains its magnetism which reheats the iron again to its control temperature.
This sudden changeover facility of the Magnastat is extremely stable, it does not deteriorate through aging or metal fatigue. The temperature reaction of the Magnastat either attracts or repels a permanent magnet, this movement is transmitted to the contact bridge switch via a push rod to switch the heating current on or off.
The switch is hermetically sealed and since the Magnastat is a part of the soldering tip, the energy loss caused through radiation or during soldering, is made up. A further advantage of this design is that the soldering iron is completely switched off while the tip is being changed so that the heating element can not burn out in the absence of a tip.