A fuse is a very thin wire, which either melts or vaporizes when too much current flows through it. The thin wire may be made of aluminum, tin-coated copper or nickel. The resulting open in the circuit stops current flow. In electronic equipment, most fuses are cylindrical glass or ceramic type with a metal cap at each end! The current rating also can be seen in one of the two metal end caps. There are two popular physical sizes: 1 – ¼ X ¼ - inch and 5X20mm. The 1 – ¼ X ¼ - inch size is used in.