Resistance is central to traditional soldering irons and to the Cold Heat iron. Electricity moves more easily through substances with lots of free electrons, like copper, than it does through substances with fewer free electrons, like carbon. In other words, substances like carbon have greater resistance. Moving current through substances with high resistance can create heat and sometimes light. This is the same principle that makes light bulbs work -- a light bulb has a resistive filament that gets hot and bright when current flows through it.
The heart of a Cold Heat tool is a broken circuit that travels from a few AA batteries to a tip that has two halves. The tip can look like one solid piece, but a dark insulating material keeps the two halves electrically isolated from one another.
When you turn the Cold Heat tool on, the switch closes a circuit that also includes a small light. This light lets you know that the tool is on. But a parallel circuit -- the one leading to the tip -- is still broken. This circuit remains broken until you put something conductive, like solder, in contact with both halves of the tip. The solder completes the circuit, also allowing current to pass through a second light.
Because of electrical resistance, both the solder and the tip heat up very quickly, and the solder melts. Dry skin doesn't conduct enough electricity to effectively complete the circuit, so the tip stays cool when you touch it.
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We'll look at the circuitry and the tip in more detail in the next two sections.