A sharp point is supposed to form a conductive corona that looks like a
dull sphere and the lightning will probably go somewhere else, but it does
not always work out that way. Lightning will sometimes hit the rod.
Whatever size wire is usually run from lightning rods will normally
survive, and that size is smaller than 3/8 inch. You get tens of
thousands of amps only for a fraction of a millisecond to around a
millisecond.
The main thing is to have the wire make as staright and smooth a run as
possible, since major sharp bends may have enough inductance to have
significant voltage drop across them from current changing by tens of
thousands of amps in a small fraction of a millisecond. Also don't have
anything conductive too close - the inductance of straght wire is
something like a microhenry every couple feet (varying slightly
inversely with the width of the wire).
In extreme cases, the voltage drop across a grounding conductor may be
enough to spark several inches and through a thin wall.
And I would not keep computers or other electronic appliances too close
to the grounding conductor of a lightning rod. I am afraid the electric
field or a sharply increasing pulse of magnetic field could zap things at
close range.
As for the rod itself: If it gets a direct hit, it will probably be not
quite the same. If it had a sharp point, it probably no longer does. It
may also be shorter, but probably by less than an inch.
Ever notice how some houses have multiple lightning rods? The reason is
that there is usually more than one surge of current over a time period of
a couple tenths of a second, and the hot air channel formed by the first
surge of current may be a few feet downwind when the next surge of current
flows.
- Don Klipstein (
[email protected])