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Early lightning research
During early investigations into electricity via Leyden jars and other instruments, a number of people (Dr. Wall, Gray, Abbé Nollet) proposed that small scale sparks shared some similarity with lightning.

Benjamin Franklin who also invented the lightning rod, endeavored to test this theory by using a spire which was being erected in Philadelphia. Whilst he was waiting for the spire completion some others (Dalibard and De Lors) conducted at Marly in France, what became to be known as the Philadelphia experiments that Franklin had suggested in his book.

Franklin usually gets the credit for being the first to perfom this experiment. The Franklin myth goes like this:

Whilst waiting for completion of the spire, he got the idea of using a flying object, such as a kite instead. During the next thunderstorm, in June 1752, he raised a kite, accompanied by his son as an assistant. On his end of the string he attached a key and tied it to a post with a silk thread. As time passed Franklin noticed the loose fibers on the string stretching out; he then brought his hand close enough to the key and a spark jumped the gap. The rain which had fallen during the storm had soaked the line and made it conductive.

However, in his autobiography, Franklin clearly states that he only performed this experiment after those made in France.

As news of the experiment and its particulars spread, it was met with attempts at replication. Experiments involving lightning are always risky and frequently fatal. The most well known death during the spate of Franklin-imitators was Professor Georg Richman, of Saint Petersburg, Russia. He had created a setup similar to Franklin's, and was attending a meeting of the Academy of Sciences, when he heard thunder. He ran home with his engraver to capture the event for posterity. While the experiment was underway, a large ball lightning showed up, collided with Richman's head, and killed him, leaving a red spot. His shoes were blown open, parts of his clothes singed, the engraver knocked out, the doorframe of the room was split, and the door itself torn off its hinges.

How lightning is formed

Multiple cloud-to-ground and cloud-to-cloud lightning strokes are observed during a night-time thunderstorm.The first process in the generation of lightning is the forcible separation of positive and negative charges within a cloud or air. The mechanism by which this happens is still the subject of research, but one widely accepted theory is the polarization mechanism. This mechanism has two components: the first is that falling droplets of ice and rain become electrically polarized as they fall through the atmosphere's natural electric field, and the second is that colliding ice particles become charged by electrostatic induction. Once charged, by whatever mechanism, work is performed as the opposite charges are driven apart and energy is stored in the electric fields between them. The positively charged crystals tend to rise to the top, causing the cloud top to build up a positive charge, and the negatively charged crystals and hailstones drop to the middle and bottom layers of the cloud, building up a negative charge. Cloud-to-cloud lightning can appear at this point. Cloud-to-ground lightning is less common. Cumulonimbus clouds that do not produce enough ice crystals usually fail to produce enough charge separation to cause lightning.

When sufficient negatives and positives gather in this way, and when the e-field becomes sufficiently strong, an electrical discharge occurs within the clouds or between the clouds and the ground, producing the bolt. It has been suggested that these discharges are triggered by cosmic ray strikes which ionise atoms, releasing electrons that are accelerated by the electric fields, ionising other air molecules and making the air conductive, then starting a lightning strike. During the strike, successive portions of air become conductive as the electrons and positive ions of air molecules are pulled away from each other and forced to flow in opposite directions (stepped channels called step leaders). The conductive filament grows in length. At the same time, electrical energy stored in the electric field flows radially inward into the conductive filament.

When a charged step leader is near the ground opposite charges appear on the ground and enhance the electric field. The electric field is higher on trees and tall buildings. If the electric field is strong enough a discharge can initiate from the ground. This discharge starts as positive streamer, and if it develops as a positive leader can eventually connect to the descending discharge from the cloud.

Lightning can also occur within the ash clouds from volcanic eruptions[1],[2], or can be caused by violent forest fires which generate sufficient dust to create a static charge.

Negative lightning

Negative C-G lightning with two visible non-connected streamersA bolt of lightning usually begins when an invisible negatively charged stepped leader stroke is sent out from the cloud. As it does so, a positively charged streamer is usually sent out from the positively charged ground or cloud. When the two leader meet, the electric current greatly increases. The region of high current propagates back up the positive stepped leader into the cloud. This "return stroke" is the most luminous part of the strike, and is the part that is really visible. Most lightning strikes usually last about a quarter of a second. Sometimes several strokes will travel up and down the same leader strike, causing a flickering effect. This discharge rapidly superheats the leader channel, causing the air to expand rapidly and produce a shock wave heard as thunder.

It is possible for streamers to be sent out from several different objects simultaneously, with only one connecting with the leader and forming the discharge path. Photographs have been taken on which non-connected streamers are visible such as that shown on the right.

This type of lightning is known as negative lightning due to the discharge of negative charge from the cloud, and accounts for over 95% of all lightning.

An average bolt of negative lightning carries a current of 30 kiloamperes, transfers a charge of 5 coulombs, has a potential difference of about 100 megavolts and dissipates 500 megajoules (enough to light a 100 watt lightbulb for 2 months).

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