On Jan 25, 10:57*am, "
wrote:
On Jan 25, 12:42*pm, wrote:





On Fri, 25 Jan 2013 07:19:46 -0800 (PST), "

wrote:
On Jan 25, 9:21*am, wrote:
On Fri, 25 Jan 2013 06:11:39 -0800 (PST), "

wrote:
On Jan 24, 9:50*pm, Red wrote:
On Jan 24, 6:38*pm, "
wrote:

On Jan 24, 5:02*pm, Erik wrote:

In article ,

wrote:
A lot of older houses had lightning rods on their roofs. *Particularly
farm houses. *They would have a thick braided cable that went to ground
rods. *Many of them had a glass ball in the center of the copper rod.

I'm wondering of they really did any good. *Sure, you want to ground a
strike, but on some ways, it would seem to me that they also could
attract lightning. *Anything that is a high point in an area attracts
it, such as large trees. *Of course metal attracts, so putting a metal
rod on a roof is an attractant. *In a direct strike, I doubt they would
do anything, in fact they might get hot enough to start a fire. *But a
discharge nearby could be sent to ground via the lightning rods.

One other thing, was there any usefulness to the glass balls in the
middle, or were they strictly decorations?

Years ago I recall watching a lightning study documentary... and IIRC,
they suggested that arrays of smaller sharp devices like aircraft static
discharge wicks might be superior.

Not that they dealt with strikes well, but were efficient at quickly
bleeding off local atmospheric electric charges, thus preventing a local
strike in the first place.

Erik

I think the idea that lightning rods bleed off atmospheric
charges has been disproven. *Conceptually it doesn't make
sense to me. *Consider two huge metal plates, say a city
block size in area, seperated by 2000 ft of air.. *You put some
metal points about a foot higher on two spots on the
bottom plate. * Now you start applying a charge to the
two plates. *Are those two points going to do anything
to lessen the charge in any material way? *Air is a pretty
good insulator, until it becomes ionized. *And once that
occurs, we know what's going to happen.....- Hide quoted text -

- Show quoted text -

Something else to consider: There's cloud-to-ground discharges and
ground-to-cloud discharges. *On a ground-to-cloud would the discharge
be from the roof rods? *I've felt a ground vibration build up to where
it rattled dishes before the discharge happened. *Thats a definite
ground to cloud discharge!

Another point though is discussion of the charge conductor - solid or
cable. *Electrical charges travel on the surface of the conductor, not
through the center.

That is true for high frequencies, not DC. *The depth the
current penetrates below the surface of a conductor
decreases as the frequency increases. *At say 60hz,
the penetration is deep enough that it's not a factor in the conductor
gauges used for typical wiring and the current
can be assumed to be carried throughout the entire
conductor.

I don't know what the frequency profile of a lightning
strike looks like, but would suspect it has a broad range
of frequency components to it, so there likely is some
skin effect involved, but I doubt anywhere near all the
current travels only close to the surface.

OTOH, like charges repell. *At these current densities it does matter.- Hide quoted text -

- Show quoted text -

Then kindly provide us a formula that shows that skin
effect applies to DC moving through a conductor at high
currents. *Or a reference that shows that this phenomena
exists. * FYI, high current does not imply a build-up of charge
within a conductor. * It's just the faster movement of
electrons that are already there.

A lightning strike doesn't add electrons to the wire? *Go figgr.- Hide quoted text -

- Show quoted text -

Not in any amounts that create the effect you're claiming.
Electrons moving through a wire are like water flowing
through a pipe. *If you double the flow rate of the water,
do you increase the amount of water in the pipe? *No you
still have the same amount of water in the pipe at any
point in time, *it's just moving through it faster
Same thing with current through a wire. *You're pushing
electrons in one end and some other electrons come
out the other end of the wire.

If you still believe you're correct, ie that skin effect exists
in a conductor for DC as current increases, it should be
easy to find the formula that expresses it. * The only skin
effect I know of is always discussed as only applying to AC
because you need a changing field to create it. *The
changing field doesn't exist with DC. *And the skin depth
effect varies as a function of the frequency and can be
expressed in a formula. * Put zero in for frequency
in the formula and you get infinite skin depth.

Disclaimer: *I'm not saying skin effect isn't of importance
in a lightning strike. *I said that I believe it is, because
lightning has a fast rise time, so clearly it has high freq
components to it. *I'm only saying DC current in a
conductor is evenly distributed.

You are absolutely correct. DC Current demonstrates NO skin effect.
The conductance carriers are distributed uniformly across the profile
of the conductor. of course, uniform in the absence of an external
field. The earth's field at 50uT is too small to notice much.

However, it is worth noting that DC current is an approximation.
Cannot truly exist because it was not always flowing. Actually, every
current is AC, just a matter of the degree of how slow you want to
consider the frequency to be.