The likelihood of lightning striking the tallest objects has been noted for thousands of years. In fact, the earliest known reference was made during the reign of Xerxes, King of Persia between 486BC and 465BC. His advisor, Artemis, wrote how bolts always struck the “highest houses and tallest trees”.
Around 400 years later, Roman poet and philosopher, Titus Carus Lucretius, mused about what is now recognised as lightning. He wrote how Jupiter left “traces of his fire on mountain tops” that had demolished holy shrines “with a devastating bolt”.
Franklin’s lightning rods
This knowledge wasn’t used to help protect against lightning strikes until 1752, when Benjamin Franklin carried out experiments in Pennsylvania, USA. He attempted to conduct lightning safely to earth via a wire – he erected an iron rod with a brass tip as part of his efforts to neutralise the electricity in thunderclouds.
The rod was struck by lightning and he believed his lightning rods would be the preferred receptor of any lightning strikes in the area, conducting the electricity harmlessly to earth through a “down conductor” driven into the ground.
After publishing his discovery in “Poor Richard’s Almanac”, Franklin’s lightning rods were installed on many buildings in America and Europe – after which, very tall structures that had been damaged by lightning several times suffered no further damage. The scheme’s success was so great that in 1778, the Senate of Venice installed lightning rods throughout the city.
Safety standards introduced
In Great Britain, some 220 Royal Navy ships were lost or damaged by lightning strikes during the Napoleonic wars of 1803 to 1815. It wasn’t until the 1830s, however, that lightning rods and down conductors were fitted on ships to stop the lightning damage.
During the early 19th century, the use of lightning protection on buildings steadily increased but there were no legal standards in place. As a result, itinerant workers known as “lightning rod men” sprang up, carrying out unskilled and often flawed installations across America.
In 1879, Richard Anderson’s book, Lightning Conductors, claimed American citizens were being “extorted” by the lightning rod men; who had no professional knowledge and were installing “untrustworthy” apparatus.
In Britain, the Royal Meteorological Society’s lightning rod conference established a code of rules for lightning protection installers that was published in 1882. Subsequently in 1904, a similar code was adopted in the USA by the National Fire Protection Association.
Since then, as knowledge and technology have advanced, the legislation has been regularly updated and is now subject to mandatory safety requirements.
Between 1907 and 1914, insurance figures show that US farm buildings with lightning protection accounted for only 1.3% of the claims, compared with unprotected buildings. Between 1943 and 1946, Army Ordnance records show that six lightning strikes destroyed explosives in unprotected munitions stores and a shipping house. Between 1944 and 1948, lightning struck protected structures more than 300 times with hardly any damage, while unprotected buildings were struck 52 times with massive losses.
Lightning protection is now routinely installed on large public buildings. The fundamental principle remains the same: they provide a means for a lightning bolt to enter the earth without damaging property.
Franklin’s rods v modern protection
Today’s lightning protection demonstrates one fundamental difference from Franklin’s early system. His lightning rods had sharp tips because he believed they could “neutralise” a thundercloud and prevent lightning. We now know this isn’t the case, as the rod functions as a strike receptor. This means a sharp tip isn’t ideal – In fact, a sharp tip would make an ineffective strike receptor. The early success of Franklin’s sharp-tipped rods occurred because they were exposed on tall structures, above any competing objects.
The best configuration of a lightning rod’s tip is now known to be blunt or knob-shaped.
Modern physics has also taught us that arcing radiates outwards from ground rods when the lightning’s current exceeds 20kA. Experts now recommend using radial conductors below ground level to distribute lightning charges to their destination – the earth’s surface.