An electrical device used to determine the location of a gap or void in the anticorrosion coating of a metal surface.
The term dates back to the days of the great wooden sailing ships. One of the most important among sailors' duties was to seal the mast of the ship with tar. if the sailor missed a spot, the wood would rot and, it was said that the sailor must have taken a holiday at that point. The term was later extended to paint on steel masts and on pipelines, and then to cable insulation.