A lightning rod is a metal strip or rod, usually of copper or similar conductive material, used to protect tall or isolated structures (such as the roof of a building or the mast of a vessel) from lightning damage. Other names include lightning conductor, lightning arrester, lightning discharger, and lightning protector.

Construction and uses

It is commonly believed, erroneously, that a rod ending in a sharp point at the peak is the best means to conduct the current of a lightning strike to the ground. According to field research, a rod with a rounded or spherical end is better. A lightning rod is connected via a low-resistance cable to the earth or water below, where the charge may be safely dissipated. Lightning rods sometimes posses a short circuit to the ground that is interrupted by a thin nonconductor over which lightning jumps. Ideally, the underground part of the assembly should reside in a muddy area, or an area that tends to become so during storms. If the underground cable will resist corrosion well, it may be covered in salt to improve its electrical connection with the ground.

In telegraphy and telephony a lightning rod is placed where wires enter a structure, preventing damage to electronic instruments within and ensuring the safety of individuals near them.

The rolling sphere technique is a method used to find the area that a conductor protects from lightning. This technique can be used to find locations where a lightning rod will protect the largest area possible.

As the stepped leader of a lightning bolt jumps toward the ground, it steps to the grounded objects nearest its path. The maximum distance that each step may travel is called the critical distance and is proportional to the electrical current. Objects are likely to be struck if they are nearer to the leader than this critical distance.

Because electricity travels along the path of least resistance, an object outside the critical distance is unlikely to be struck by the leader if there is a grounded object within the critical distance. Noting this, locations that are safe from lightning can be determined by imagining a leader's potential paths as a sphere that rolls until it is blocked by grounded obstacles. The leader is most likely to jump to objects touching the sphere. Areas that the sphere cannot roll across are safest from lightning. Lightning rods should be placed where they will prevent the sphere from touching a structure.

History

United States

In the United States, the lightning rod was invented by Benjamin Franklin as part of his groundbreaking explorations of electricity. Franklin speculated that with an iron rod sharpened to a point at the end "the electrical fire would, I think, be drawn out of a cloud silently, before it could come near enough to strike...." Franklin had speculated about lightning rods for several years before his reported kite experiment. In fact, the experiment took place because he was tired of waiting for Christ Church in Philadelphia to be completed so he could place a lighting rod on top of it.

There was some resistance from churches who felt that it was defying divine will to install these rods. Franklin countered that there is no religious objection to roofs on buildings to resist precipitation, so lightning, which he proved to be simply a giant electrical spark, should be no different. In the 19th century the lightning rod became a symbol of American ingenuity and a decorative motif. Lightning rods were often embellished with ornamental glass balls (now prized by collectors) that also served to provide visual sign of a lightning strike (when the rod is struck the glass ball shatters and falls off, indicating to the owner which rod got struck and that they should check it and the grounding wire for damage). The ornamental appeal of these glass balls has also been incorporated into weather vanes.