Heating Elements and Temperature Switches
Curling irons have heating elements in them, which are pieces of metal that conduct electricity very slowly, getting steadily hotter from the resistance that builds up while the electricity tries to pass through the element. They also have a temperature switch, which is the device that activates and deactivates the heating element, and of course, a power switch. There is a thermal switch inside the device that is constructed of two different metals, which conduct electricity at two different speeds and cause the switch to temporarily warp or become misshapen, resulting in a bulging chip of metal that no longer achieves contact with the electrical current, thereby shutting off the heating element.
An Electrical Circuit
Once the element cools down sufficiently, the two metals come together again to reconnect the circuit, causing the electricity to flow once again and the element re-heats. In this fashion, the curling iron heats, cools, and heats again, never getting hot to the point where it will scorch human hair. When the iron is activated but not currently in use it can be balanced on a countertop, as long as it does not come into contact with anything that may melt.
Different things melt at different temperatures, so what does not melt human hair just might melt that plastic toothbrush cup that is resting too close to the hot iron. It is of ,course, important to remember to not touch the hot end of the wand with your dry, bare hands as burns are likely. Most curling irons have a temperature gauge on the barrel that changes color when the iron is the correct temperature or a light that burns only when it is sufficiently hot, so that will indicate when the iron is ready for use.