Lightning rod may be a laser

Eripuit coelo fulmen, ("He used to snatch the lightning from the sky") they said of Benjamin Franklin

Eripuit coelo fulmen, ("He used to snatch the lightning from the sky") they said of Benjamin Franklin. The reference, of course, was to his invention of the lightning rod some 250 years ago to prevent tall structures from being struck.

More recently, however, efforts are being made to harness laser technology to do the same thing more effectively.

Until Franklin came along, fervent prayer was the only remedy against the serious damage caused by lightning, although in medieval times it was the general practice to supplement the power of prayer by the violent ringing of church bells. It was believed that this pious exercise dispersed the evil spirits of the storm.

Lightning occurs following the build-up of a very large negative electrical charge near the bottom of a thundercloud, and a corresponding positive charge on the surface of the Earth below. When the resulting electrical tension is sufficiently great, a lightning stroke is initiated, essentially a brief surge of electric current through the atmosphere which neutralises the opposing charges.

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Franklin's device, a sharp rod fixed to the highest point of a building, and connected to the earth by means of a stout strip of iron or copper, provides the easiest path in the vicinity for the surge of energy; once the path is established, the electric current passes harmlessly to earth through the metal strips, without any damage to the structure.

Now a laser produces a very powerful, finely focused beam of light of a specific wavelength, and has the capability of dislodging negatively charged electrons from some of the atoms in its path. As a result, after a laser pulse has been aimed upwards at a thundercloud, the path it occupied contains a large number of electrically charged particles, and this "ionisation" of the air provides a conducting path to facilitate a flow of current.

The idea is to use the laser to discharge a cloud harmlessly, by means of a guided, controlled stroke, before the electrical tension becomes strong enough for a random surge of lightning to take place.

To prevent the laser itself from being struck, the beam is not pointed directly at the cloud; it is aimed at a mirror, suitably positioned on the ground and well earthed, which in turn redirects the laser beam to the chosen thundercloud.

When perfected, it is hoped that a portable version of this device might be suitable for temporary installation at sporting events, or at other places where the population might be vulnerable.