A soldering iron is a hand tool most commonly used in soldering. It supplies heat to melt the solder so that it can flow into the joint between two workpieces.
A soldering iron is composed of a heated metal tip and an insulated handle. Heating is often achieved electrically, by passing an electric current (supplied through an electrical cord or battery cables) through the resistive material of a heating element. Another heating method includes combustion of a suitable gas, which can either be delivered through a tank mounted on the iron (flameless), or through an external flame.
Less common uses include pyrography (burning designs into wood) and plastic welding.
Soldering irons are most often used for installation, repairs and limited production work. High-volume production lines use other soldering methods. 
For electrical work a low-power iron, a power rating between 15 and 30 watts, is used. Higher ratings are available, but do not run hotter; instead there is more power available for larger joints.
Small battery-operated or gas soldering irons are useful when electricity is unavailable.
Temperature-controlled soldering station
A temperature-controlled soldering station consists of an electrical power supply and a soldering iron. It is most commonly used for soldering electronic components.
A variety of means are used to control temperature. The simplest of these is a variable power control, much like a light dimmer, which together with the loss of heat from the iron to the environment roughly sets the temperature. Another type of system uses a thermostat, often inside the iron's tip, which switches power on and off to the elements. A more advanced version of this uses a microprocessor to monitor the temperature of the tip via a thermocouple and adjusts the power to the heating element accordingly.
Another approach to use magnetized soldering tips which lose their magnetic properties at a certain temperature (the Curie point). As long as the tip is magnetic, it clings to the heating element. At the design temperature, it loses contact, cooling down. Other complex irons circulate a high-frequency AC current through the tip, using magnetic physics to direct heating only where the surface of the tip drops below the Curie point.
A soldering iron stand keeps the iron away from flammable materials, and often also comes with a cellulose sponge and flux pot for cleaning the tip. Some soldering irons for continuous and professional use come as part of a soldering station, which allows the exact temperature of the tip to be adjusted, kept constant, and sometimes displayed.
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