The Extech ET40B Compact Size Continuity Tester with Stainless Steel Probe is a main unit that detects open circuit in fuse, diodes, switch, and wires. The ET40B features a built-in 2.9" (73mm) stainless steel test probe with 4.9ft (1.5m) cable, as well as an insulated alligator clip, and AAA battery.