A lightning rod or lightning conductor (British English) is a metal rod mounted on a structure and intended to protect the structure from a lightning strike. If lightning hits the structure, it will preferentially strike the rod and be conducted to ground through a wire, instead of passing through the structure, where it could start a fire or cause electrocution. Lightning rods are also called finials, air terminals, or strike termination devices.
In a lightning protection system, a lightning rod is a single component of the system. The lightning rod requires a connection to the earth to perform its protective function. Lightning rods come in many different forms, including hollow, solid, pointed, rounded, flat strips, or even bristle brush-like. The main attribute common to all lightning rods is that they are all made of conductive materials, such as copper and aluminum. Copper and its alloys are the most common materials used in lightning protection.
The principle of the lightning rod was first detailed by Benjamin Franklin in Pennsylvania in 1749, who in subsequent years developed his invention for household application (published in 1753) and made further improvements towards a reliable system around 1760.
As buildings become taller, lightning becomes more of a threat. Lightning can damage structures made of most materials, such as masonry, wood, concrete, and steel, because the huge currents and voltages involved can heat materials to high temperature. The heat causes a potential for structure fire, and its rapidity can lead to explosive damage as well.
A lightning conductor may have been intentionally used in the Leaning Tower of Nevyansk. The spire of the tower is crowned with a metallic rod in the shape of a gilded sphere with spikes. This lightning rod is grounded through the rebar carcass, which pierces the entire building.
The Nevyansk Tower was built between 1721 and 1745, on the orders of industrialist Akinfiy Demidov. The Nevyansk Tower was built 28 years before Benjamin Franklin's experiment and scientific explanation.