On 1/23/2013 7:07 AM, Tomsic wrote:
wrote in message
...
On Jan 23, 5:54 am, 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?

They do not "attract" lightning unless the charged area is within
a few feet of them already, solely because they provide a slightly
shorter path than the surrounding protrusions.
Electric fields appear randomly, and move across the terrain.
When the jump path is less than required for the discharge voltage,
it flashes over. If the field moves over a house, the lightning rod
defines the jump path, and discharge thru the house is avoided.

The glass ball is decoration. Makes it look like the customer
is paying for more than a thick piece of metal. The metal needs
to be set above the roof with an insulator, until it terminates in
the ground.

Telephone poles usually have a piece of copper wire running
down them to the ground, for the same reason.

This technique is also used in some types of aircraft and in
missles, which are often struck by lightning. The metal strips
act as a guide to channel the bolt in a particular path, and
prevent it from jumping willy-nilly into anything that may be
around, which could cause damage. A metal fuselage will
take care of itself, but a composite fuselage , or long
radome, needs "channels" for this.

Lightning is very dangerous for flagpole sitters.

Because the flow of the electric fields during a thunderstorm is so
unpredictable, the best that lightning rods can do is reduce the chances of
a damaging strike. That they do. What lightning rods also do, and not
mentioned above, is to bleed the ground charge into the air harmlessly
through the needle-sharp tip of the rod. That reduces the voltage between
the ground and the cloud overhead and so further reduces the chances of a
lightning strike.


A properly installed lightning rod (now called "air terminal") system
provides very good protection to a building. They provide a safe "point
of attachment" for lightning that will hit the building anyway. The do
not attract lighting and "metal" does not attract.

As electrical charge descends in a "stepped leader" lightning will hit
what is nearest to the leader when the leader is close enough to the
ground. The rod system is designed so the rods are nearer than the building.

They do not "bleed the charge", and points are not necessarily
needle-sharp. The rather limited research that has been done shows a
rounded tip is more likely to be hit than a sharp one at the same
location. A sharp one will be hit if there is no rounded one.

The rod system is not insulated from the building. It may have to be
bonded to metal within 6 feet of the rods and down conductors. If not
bonded, the rod system can be at high enough potential with respect to
other building parts there can be a side flash during a lightning strike.

A properly installed system also includes other protection, including
surge protection at the power service panel.