During a thunderstorm charges collect on clouds until a field which is more than the air's dielectric strength is created. Because now there is a potential difference between the ground (0 Volts) and the clouds, electric current flows through the now conducting air. If there is a building, it is conducting and thus at same potential as the earth - as is its lightning conductor. So when there exists a potential difference between the clouds, and the lightning conductor and building both, why does current only flow through the former? My teacher said it's because the lightning conductor is closer to the clouds, so the resistance of the conducting medium air (which is proportional to length) is lesser, the current that flows through to the conductor is greater so it conducts away most of the charge. But the conductor is not very much closer to the cloud as compared to the building - it's only a bit taller. Also the charges on the cloud I think are too high to be conducted away quickly by just the conductor so none are left over for the building to conduct - the charges will I think (if this were the real explanation) flow through both the same time and not wait till most was conducted away by the conductor. What exactly is the working of a lightning conductor?
The lightning conductor provides a very low resistance path between the top of the lightning conductor rod via a substantial strip of metal to the ground so that the current as a result of a lightning strike does not flow through the building to which it is attached.
However that is not the end of the story in that the electrical length of the lightning conductor rod can be thought of a being larger than its physical length.
A large electric field will make air become a conductor and the electric field around sharp objects is larger than that where the surface is flatter.
Around the top of a pointed lightning conductor rod charges are induced by the charge on the clouds above.
When the induced charge is sufficiently large the air around the point of the lightning conductor becomes conducting thus extending the effective length of the conducting path of the lightning conductor.
Modern lightning conductor rods are now made with slightly rounded points because it has been found that although the electric field produced by a charged rounded point as compared with a changed sharp point is smaller the region over which the air is a conductor is larger. So air is conducting over a larger distance above the lightning conductor rod.
Of course the ionisation of the air around a lightning conductor rod increases the chances of lightning striking the rod but that is considered to be safer to have the lightning conductor rod than just allowing the lightning strike in a more random fashion.
There is another benefit of the lightning conductor rod in that in making the air around it a conductor it can thus contribute to the discharge of the clouds without lightning being produced and so reduce the chances of a lightning strike..
The highly charged clouds, charged with high density of electrons on the outer layers of the clouds (in ionised state), induce opposite charge around the lightning conductor almost equally opposite to the potential of charge in the cloud. This makes the lightning to get attracted towards the accumulated opposite charges around the lightning conductor, which, in a way, creates a virtual path for the lightning to drain out the high potential electric charge along the conductor.