Tuesday, 12 May 2015

The lightning Rod


The lightning rod is a device to attract and disperse atmospheric electrical discharges. It was invented by Benjamin Franklin, American physicist, and was applied for the first time successfully in Paris May 10, 1752.

To get to the lightning rod Franklin had made some important considerations on atmospheric electrical discharges, coming to establish that the damage caused by these were not due so much to their power, how much heat they generated on impact with any object. Also he found that when lightning, which is nothing more than an electrical discharge, striking an object, passes through only part: it was necessary then to think of something that would attract the lightning and disperse them through the force of a fixed course.

Discovering the peculiarities of metal spikes, especially those in gold, to attract the electrical discharges, thus acting as a magnet against the lightning, Franklin solved the biggest challenge: to capture him during his discharge.

 The lightning rod then consists in a long, thin metal rod coated with the tip of a noble metal (intrinsically devoid of surface layers of oxides and therefore high electrical conductivity) placed on the top of the building to be protected; from this it is derived a metal wire which is connected to earth: the electrical discharge is drawn from the tip and dispersed to the ground by means of wire. Moreover, the lightning rod, because of its shape, also has a preventive action against lightning. This consequently to the fact that the ground and the lightning conductor (connected to ground) are polarized by induction in response to the charge present on the bottom of the cloud. The lightning conductor so polarized, thanks to the dispersing power of the tips, helps to decrease the potential difference between the cloud and the ground, making it less likely that it reaches the minimum potential capable of beginning the download.

Due to the high currents and voltages that pass through it, the cable for the discharge of lightning can not be shielded. Generally copper, must have an adequate thickness to prevent current leakage into the surrounding space. For the same reasons, the link between the tip of the lightning rod and the grounding should be as short as possible, preferably in a straight line in particular avoiding sudden bends or turns that would increase the electrical impedance.