In simplest terms, welding is a process by which two or more pieces of metal are joined by applying heat and pressure. In the past, blacksmiths and other crafty people would heat metals in a furnace and then weld them by hammering the red-hot metals together. By hammering the metals as they cooled, the weld would be made stronger. This heating-and-hammering method is known as forge welding.

While forge welding worked quite well for most of the welding done back then, today's welding requirements are a bit more advanced. After all, it would be pretty difficult to heat all the metal needed to build an automobile in a big factory furnace and expect workers to hammer together each specific part used in the manufacturing process. We'd all still be riding horses to work!

In 1885, Elihu Thompson invented a process called electric resistance welding. He discovered that to weld metals together, one could fire an electric current through the metals while they were tightly clamped together. When the current passed through the metals, it would create such a high heat that the metals would melt and run together and a weld would be made. Many times, the welded metal would be even stronger than the original metals used in the welding process.

Today's resistance welders work almost exactly the same way they did when Thompson invented the process. The current is generated by a transformer, and is fired through electrodes, which hold the metal pieces in place. These electrodes also apply force to the metal pieces, usually before, during, and after the firing of the electric current. This method is called resistance welding because it is the resistance between the contact surfaces of the metals being welded that generates the heat to fuse them together.