What are fuses and why do they come in different sizes?

Question

Fuses are devices placed in electrical circuits as protection for those circuits. They consist of small pieces of wire, sometime as ribbons, and are placed in series with the circuit so that the current passing through the device must pass through the fuse. A device that draws, say 1 ampere of electrical current may have a 2 ampere fuse in it. The device works ok unless and until something happens that causes more than 2 amperes to flow through the circuit. At that point, the fuse literally melts, stops conducting the electrical current, and the system shuts down. But suppose we have a device that uses 8 amperes. Then we must use a larger fuse; i.e., a 2 ampere fuse would be no good because it would not allow the device to operate. An air conditioner may use 20 amperes so we would put a slightly larger fuse to operate that. That in a nutshell is why fuses come in different sizes; i.e., we pick a size that fits the job we want to do.