Why do lightning rods have a sharp point at the top?

We know that a lightning rod or lightning conductor is a metal rod or metallic object mounted on top of an elevated structure and, if we look closely, most of them have a sharp point at the top. What is the reason for this sharp point?

• Have you read this pat of the wiki en.wikipedia.org/wiki/… – Azad May 16 '15 at 14:30
• It increases surface charge density at that small pointed end, ${\sigma}=\frac{q}{4\pi r^2}$ for it $r$ is quite small and if we consider an spherical approximation $dE = \frac{\sigma}{\epsilon_0}$ , this enhances the discharge phenomenon. – Mann May 16 '15 at 14:31
• @Mann Please convert that comment to an answer. – rob May 16 '15 at 15:26
• I suspect the real answer might be either "luck" or "because it seemed to help", that is to say the practice of doing it came experimentally, before the theory to explain why it's a good idea. – Steve Jessop May 16 '15 at 18:53
• Although it doesn't go into technical details, you may enjoy this video: Should a Person Touch 200,000 Volts? (the bit relevant to this question starts at around the 4:00 minute mark, and continue towards the end of the video) – IQAndreas May 16 '15 at 21:56

Suppose there is a charged cloud floating over your conductor. Then making your lightning conductor pointy at the edge would facilitate better discharge by setting up a high electric field.

We will take a spherical approximation of the pointed end, then $${\sigma}=\frac{q}{4\pi r^2}$$ is the surface charge density of the end. It has a very high surface charge density due to its small radius. Hence, in this case, the electric field over that small part will be $$E=\frac{\sigma}{\epsilon_0}$$ which is also very high.

Then, for a pointy metal rod, the electric field set up at pointy ends is high. Now for some reason, if the discharge of the cloud occurs, the charge will be easily passed through the lightning conductor and conducted to the ground. Your artifact which you are trying to save is ultimately protected from damage.

• Thank you for converting your comment to this nice answer! – rob May 16 '15 at 16:26
• Quick and better :) +1 – Shashank May 16 '15 at 16:55

The point of the point is to increase the electric field near the point. Small radius curves will have a higher local electric field, eventually creating a localize area where the field is greater than the dielectric strength of the air. This results in what I refer to as "micro-lightning." This microlightning discharges the air (or cloud) before the charge difference between the cloud and ground builds to the point where a very long path of breakdown is formed. The main idea is to prevent big lightning by have near-continual (during storms) microlightning.

You can demonstrate this with a small Tesla coil or classroom Van de Graaff generator. Set up a situation with the coil or generator causing long (>10 cm) sparks. Then get a pointy object like a key or a nail, ground it, and bring it near the discharge. The spark will stop, but if you listen carefully, you can hear a crackle near the pointy object. You won't get a large spark around the pointy object until you get close to the coil tip or generator sphere. Then remove the pointy object and the long sparks will start again.

This section might help:

http://en.wikipedia.org/wiki/Lightning_rod#Should_a_lightning_rod_have_a_point.3F

Which also states that :

Finding that moderately rounded or blunt-tipped lightning rods act as marginally better strike receptors.

Research conducted with actual lightning demonstrates that blunt lightning rods are as effective as pointed (tapered) rods. This has been codified in the National Fire Protection Association standard NFPA 780 - Installation of Lightning Protection Systems. The blunt tips are also pose less risk to someone who might fall while working on a roof.

If there is an excess charge in the atmosphere, as happens during thunderstorms, a substantial charge of the opposite sign can build up on this blunt end. As a result, when the atmospheric charge is discharged through a lightning bolt, it tends to be attracted to the charged lightning rod rather than to other nearby structures that could be damaged. (A conducting wire connecting the lightning rod to the ground then allows the acquired charge to dissipate harmlessly.) A lightning rod with a sharp end would allow less charge buildup and hence would be less effective.

protected by Qmechanic♦Jan 2 '17 at 6:42

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