Lightning rod
From Wikipedia,

A lightning rod (or lightning protector) is a metal strip or rod, usually of copper or similar
conductive material, used as part of lightning safety to protect tall or isolated structures
(such as the roof of a building or the mast of a vessel) from lightning damage. Its formal
name is lightning finial or air terminal. Sometimes, the system is informally referred to as:

a lightning conductor,
a lightning arrester, or
a lightning discharger.
However, these terms actually refer to lightning protection systems in general or specific
components within them.

Construction and uses

A lightning rod is connected via a low-resistance wire or cable to the earth or water below,
where the charge may be safely dissipated. Lightning rods sometimes possess a short circuit
to the ground that is interrupted by a thin non-conductor 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.
Similarly, high-tension power lines carry a lighter conductor wire over the main power conductors.
This conductor is grounded at various points along the link. Electrical substations usually
have a web of the lighter conductor wires covering the whole plant.

Considerable material is used in the construction of lightning arresters, so it is prudent to
work out where a new arrester will have the greatest effect. Historical understanding of lightning
assumed that each rod protected a cone of 45 degrees [1]. This has been found to be unsatisfactory
for protecting taller structures, as it is possible for lightning to strike the side of a building.

A better technique to determine the effect of a new arrester is called the rolling sphere technique
and was developed by Dr Tibor Horváth. To understand this requires knowledge of how lightning
'moves'. As the step leader of a lightning bolt jumps toward the ground, it steps toward 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. It is standard practice to approximate
the sphere's radius as 60m near the ground.

Electricity travels along the path of least resistance, so 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 travels from the cloud to the ground.

For lightning protection it suffices to consider all possible spheres as they touch potential
strike points. To determine which strike points consider a sphere rolling over the terrain.
At each point we are simulating a potential leader position and where the sphere touches the
ground the lightning is most likely to strike. Points which the sphere cannot roll across and
touch are safest from lightning. Lightning rods should be placed where they will prevent the
sphere from touching a structure.

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. "Lightning Rod Improvement Studies" [2] by
Moore et al say:

Calculations of the relative strengths of the electric fields above similarly exposed sharp and
blunt rods show that although the fields, prior to any emissions, are much stronger at the tip
of a sharp rod, they decrease more rapidly with distance. As a result, at a few centimeters above
the tip of a 20-mm-diameter blunt rod, the strength of the field is greater than that over an
otherwise similar, sharper rod at the same height. Since the field strength at the tip of a
sharpened rod tends to be limited by the easy formation of ions in the surrounding air, the field
strengths over blunt rods can be much stronger than those at distances greater than 1 cm over
sharper ones. The results of this study suggest that moderately blunt metal rods (with tip height–to–tip radius
of curvature ratios of about 680:1) are better lightning strike receptors than are sharper rods
or very blunt ones.

History
Wooden churches with lightning rods and grounding cables. Lightning damage has been with humanity
since we started building structures. Early structures made of wood and stone tended to be short
and in valleys and as a result lightning hit rarely. As buildings became taller lightning
became a significant threat. Lightning can damage structures made of most materials (masonry,
wood, concrete and even steel) as the huge currents involved can heat materials, and especially
water to high temperatures causing fire, loss of strength and explosions from superheated steam
and air.

Europe
The church tower of many European cities, usually the highest structure, was the building often
hit by lightning. Early on, Christian churches tried to prevent the occurrence of the damaging
effects of lightning by prayers. Priests prayed,to temper the destruction of hail and cyclones and
the force of tempests and lightning; check hostile thunders and great winds; and cast down the spirits
of storms and the powers of the air. Peter Ahlwardts ("Reasonable and Theological Considerations
about Thunder and Lightning", 1745) gave information to individuals seeking cover from lightning to
go anywhere except in or around a church.[3]

United States
In the United States, the pointed lightning rod conductor, and more accurately the "lightning attractor",
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.

This experiment, in fact, 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[4] (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.

As a point of fact, as an act of philanthropy, Benjamin Franklin decided against patenting the
invention