Jack Painter wrote:
"Your example of lightning was the opposite, and very rare, from the
normal occurance of lightning, which is not positively charged."

I won`t predict nor defend either polarity of charge in an overhead
cloud. In either case it will attract the opposite charge beneath.

In my example, I chose positive for the cloud because now it is popular
to characterize electrons as the mobile electrical particle. My formal
education was that regardless of what electrons do, current flows from
plus to minus. In my hypothesis, I chose to characterize what charges
were doing in the earth and other conductors. Electrons are considered
mobile due to extremely small mass, so I chose them as the moving
charge.

Effects of lightning are identical regardless of polarity or direction
of travel for practical purposes. Significant factors in lightning
protection are the maximum voltage reached across a protected device and
the current through it. We can`t do anything about the millions of volts
and the thousands of amps behind a lightning strike. We can put high
impedance between the strike and our protected people and products and
we can clamp the volts across them.

I`m sure that polarity makes no difference, one, the other, or both. For
sure you must be prepared for both because lightning is a transient. In
its start and stop, it generates powerful alternating components which
include the entire radio spectrum.

Arc gaps are effective protectors which have dependable breakdown
voltages for a given atmosphere, temperature, pressure, presence or
absence of triggering radiation, etc. Breakdown voltage is proportional
to gap width for given conditions. Electrode shape is significant in the
striking (breakdown) voltage. Blunt electrodes require significantly
more volts to arc across than do needle points. Anyone in the TV
business knows that corona discharge from the high-voltage circuit
occurs from a sharp point. Once a corona is producced from a point,
conductance extends as far as the corona reaches.

I agree that charged air arrives in advance of a thunderstorm, whatever
the polarity of charges may be. I`ve spent many years in broadcast
stations and observed the storms. Before the storm arrives, tower guy
insulators flash across from charges picked up by the guywire segments
out of thin air.

On arrival, a bolt from the sky hits a tower or towers. Maybe it`s the
reverse that occurs. It makes no difference. The station and its
equipment are protected by the tall towers. In lightning rods, more is
better. All the stations in my experience used multitower arrays. None
ever had significant lightning damage. Much of the time they were
operating 24-7 and they might be dropped from the air for an instant due
to a temporary overload caused by the lightning strike.

The medium wave stations I worked in had blunt lightning discharge
electrodes. Lowest breakdown voltage wasn`t a requirement. The final
amplifiers used vacuum tubes and were sturdy. The tower base insulators
all had ball gaps side by side across them. Precipitation fell right
through to have little effect on their striking voltage. Tower balls
never fire anyway. It`s always the Faraday screen between the primary
and secondary of the tower coupling transformer thet takes the lightning
hit. The impedance and striking voltage are higher at that circuit
point. The screen is heavy and easily withstands lightning strikes.

The shortwave stations I`ve worked in, all had arc gaps at the feedpoint
of their parallel 600-ohm transmission lines. These were adjusted so
that they just had 10 thousandths of an inch more than the minimum
spacing required to prevent breakdown on full 100 KW carrier output with
100% modulation. These gap electrodes were consstructed like the letter
V turned so one side was horizontal and the pointed ends of the V`s
pointed to each other. The upward slope of the arc gap meant that heated
air in the gap would rise increasing the arc distance so it might
self-extinguish. We never had any lightning damage in the short wave
plant with its dozens of antennas, lines and transmitters.

Sharp points lower breakdown voltage. In a situation where charge across
a capacitance, auch as between a cloud and the earth, has increased to
the breakdown value, it happens where there there is a sharp point in
the neighborhood of the charge concentration.

Blunt electrodes require higher potential to flash over than sharp
electrodes. For given electrodes in a controlled environment, the
distance between electrodes can be calibrated in volts required to jump
the gap.

Arc gaps are spaced to clamp the maximum voltage across them to a safe
value. CRC`s "Handbook of Chemistry and Physics has a table of the
voltages for blunt and sharp electrodes.

Best regards, Richard Harrison, KB5WZI