Johnson's plumb bobs are ideal tools for finding a plumb line, or true vertical, over a long distance. These brass plumb bobs have corrosion resistant lacquer finish for long life. Features a removable cap for quick, easy and accurate centered string installation. To attach the string, simply unscrew the cap, thread the string through top hole and tie a knot. To use, hold near top of string or attach to a nail. When the string stops moving, its position is true plumb.