Re: Lightning Generator

To: tesla-at-poodle.pupman-dot-com
Subject: Re: Lightning Generator
From: Tesla List <tesla-at-stic-dot-net>
Date: Wed, 07 May 1997 00:33:34 -0500
Approved: tesla-at-stic-dot-net

Subject:     Re: Lightning Generator
      Date:  Tue, 06 May 1997 19:20:13 -0800
      From:  Greg Leyh <lod-at-pacbell-dot-net>
        To:  Tesla List <tesla-at-pupman-dot-com>
References: 
           1

Thomas McGahee wrote:

> Ed,
> It is a single polarity, true... but what makes something RF is the
> "apparent" frequency of a 1/2 cycle, which of course is a unipolar
> definition, is it not? Some Tesla coil builders use a DC supply to
> supply raw power to the tank circuit. The gap fires and we call it
> RF. And to take it to its extreme, a Tesla coil operated in ONE SHOT
> mode only gets a single half cycle of excitation. But it STILL
> operates as a resonant RF circuit anyhow.

A lightning discharge gets it RF frequencies not from a resonant
circuit, but rather from an RC relaxation oscillator circuit.  
A typical strike actually advances towards the ground in a series 
of discrete steps, which are called 'dart leaders'.

Each dart leader builds on to the end of the main strike, adding
anywhere from 30 to 200 feet to the total strike length.  The leader
temporarily depletes the local charge in the cloud at the base of
the main strike, requiring some time to pass before the charge can
build up far enough to trigger the next leader.  Typical times 
between leader firings can range from 8 to 100uS (10kHz to 120kHz), 
resulting in a rich RF content during the strike.

The TC arc advances in a similiar way -- not as a single arc, but
rather as a series of streamer advances, each firing of the gap
adding an incremental amount to the existing ionization path from
the previous gap firing.  The TC streamers repeat at a slow enough
rep rate (the gap rate) that their progress can be viewed using
a high quality video tape, in frame-by-frame slow motion.

-GL