Lightning conductor

How the charge builds up. Inside a storm cloud, strong rising and falling air currents drag water droplets and ice particles against one another at high speed. This repeated rubbing separates charge in the same basic way that charging by friction separates charge on a comb or a balloon — except on a scale of millions of tonnes of moving air and water. Negative charge tends to collect on the lower part of the cloud, and positive charge on the upper part, and correspondingly on the ground beneath. As the storm continues, the amount of separated charge keeps growing, and so does the electric stress between cloud and cloud, or between cloud and ground.

Lightning is the discharge. Once the charge built up on one side becomes large enough, the surrounding air — normally an insulator — breaks down along a narrow channel, and a massive discharge current flows in a fraction of a second. This sudden flow of charge is lightning: a blinding flash of light from the air heated to tens of thousands of degrees, followed by the crack and rumble of thunder, the sound of that superheated air expanding explosively. A single lightning strike can carry tens of thousands of amperes of current, though it lasts only a tiny fraction of a second.

Why tall, isolated structures get struck. Electric charge always looks for the shortest, easiest path between two oppositely charged regions. A tall building, a tower, or a lone tree standing above its surroundings shortens the gap between a charged cloud base and the ground far more than the flat land around it does. This is exactly the same idea that makes a lone tall tree in an open field more likely to be struck than the low grass around it — height and isolation both increase the chance of being the point where the discharge finally breaks through.

What the conductor does about it. A lightning conductor does not stop a charged cloud from discharging near a tall building; nothing can prevent that. What it does is control where the current goes once the strike happens. It typically has three connected parts: a pointed metal rod or a cluster of spikes fixed at the highest point of the structure; a thick metal strip (usually copper or aluminium) running down the outside of the building; and a metal plate buried deep in moist earth, which conducts well because wet ground has plenty of dissolved ions to carry charge. Because metal offers far less resistance to current than brick, wood, or plaster, a strike on the conductor flows almost entirely down this external metal path and into the ground, largely bypassing the structure's own walls, roof, and internal wiring.

Every path a lightning discharge could take has some resistance, and current strongly prefers the path of least resistance. Brick, timber, and plaster all resist charge far more than a continuous metal strip does, so a properly fitted lightning conductor draws an overwhelming share of the current down its own external path rather than through the building's fabric.

Step 1. The pointed rod is fixed at the highest point of the structure, the point closest to the charged cloud base and so the most likely spot for a discharge to make contact. A sharp metal point also lets charge on the structure leak away gradually into the air, the way a pin held near a charged object slowly drains it, easing (though never fully removing) any sharp build-up on the building.

Step 2. Once a strike hits the spike, the thick metal strip along the outside wall — a far better conductor than the masonry beside it — carries almost all the current down to ground level instead of letting it arc through the interior, where it could ignite fires or damage wiring.

Step 3. At the bottom, the strip meets a metal plate buried in moist soil. Moist earth conducts well because dissolved salts and minerals let current spread out and disperse harmlessly into a large volume of ground, rather than staying concentrated at one point.

Step 4. The structure is never made "strike-proof" — a conductor cannot lower the chance of a strike happening nearby. What it changes is the outcome: because the path from spike to earth is metal and continuous, the current is channelled safely away from people and property inside.

Question. A school building has a lightning conductor fitted: a metal spike at the roof, a copper strip down the outside wall, and a plate buried in moist ground near the foundation. During a storm, lightning strikes the spike. Explain, step by step, what happens to the discharge current, and why the classrooms inside remain safe.

Step 1. The spike, being the highest and sharpest point of the building, is the most likely place for the cloud's discharge to make contact, so the lightning strikes there rather than at a random point on the roof or walls.

Step 2. The copper strip fixed to the outside wall offers a continuous, low-resistance metal path from the spike down to ground level, so almost all of the current flows down this strip rather than through the brick walls or the timber roof structure.

Step 3. At the base, the strip is connected to a metal plate buried in moist earth, which conducts the arriving current away and disperses it into the ground, so charge does not accumulate anywhere near the building.

Step 4. Because the current's entire journey — from the strike point to the ground — stays on this dedicated metal path, it never has to pass through the classrooms, wiring, or occupants. The lightning conductor does not stop the strike, but it keeps the resulting current away from the building's structure and people, so the classrooms remain safe.

Competitive stretch

Now suppose the same storm passes over an open field beside the school, where a single tall tree stands alone. Explain why a person should not shelter under it. The tree is tall and isolated, so it is a likely strike point for the same reason the school's spike is — height and isolation shorten the gap to the cloud. But the tree has no conductor: there is no low-resistance path carrying a strike safely to ground. If lightning strikes it, the current travels through the trunk (which can char or split) and can spread outward through the ground near its base, close enough to reach a person standing nearby. Sheltering under an isolated tree puts a person beside its own uncontrolled discharge path — exactly the danger a lightning conductor is designed to remove from a building.

• Thinking a lightning conductor stops lightning from striking a building — it does not reduce the chance of a strike; it only gives the resulting current a safe path to ground.
• Assuming any metal rod fixed to a roof counts as a working lightning conductor — it must form one unbroken, well-connected conducting path from the rooftop spike to a buried earth plate, or it cannot do its job safely.
• Believing any tree offers safe shelter in a storm — a single tall, isolated tree is actually more dangerous than open low ground, precisely because its height and isolation attract a strike.
• Mixing up a lightning conductor with ordinary household earthing — both carry charge safely to the ground, but a lightning conductor is built to survive a massive, brief lightning discharge, while household earth wires are designed for small, everyday fault currents in appliances.

Ask the tutor: Want to work through why a person crouching in an open field is safer than standing under a tree, or how real buildings are fitted with lightning protection?