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RE: Terry's Test - Two Manifestations of Charge

Original poster: Jim Lux <jimlux@xxxxxxxxxxxxx>

At 04:04 PM 7/8/2005, Tesla list wrote: 
Original poster: Terry Fritz <teslalist@xxxxxxxxxxxxxxxxxxxxxxx>


It is also interesting that a 38% increase in current makes the
ground spark seem much brighter.  I wonder if the luminosity of
the spark increases only by 38%, or if it is much higher?

It might be a current squared function. In that case the spark would be 90% brighter. But I am not sure the current to brightness curve is linear especially if we are going from the purple to white transition region.

To a first order, the total light energy will be proportional to the energy dissipated in the spark. (that is, sparks are a pretty efficient "light bulb", which is why xenon short arc lamps are popular) Here's the gotchas:

1) The spectral sensitivity curve. Your eye, for instance, is less sensitive to blue than to green light. Oddly, (or not so oddly), your eye's spectral response matches a black body at around 5500K (the apparent black body temperature of the sun).

2) Time integration. Your eyes, CCD sensors, and photographic film all have different sensitivities depending on the length of exposure (called reciprocity failure in film).

3) Apparent brightness is related to apparent size of the source, which will be affected by the current in the spark.


In general, spark channel diameter grows as the square root of current (that is, the current density tends to remain constant). But, there's also an energy balance issue.. Bazelyan and Raizer go into all this in some detail for the dynamic case. Earlier works such as those by Cobine (Gaseous Conductors) and Somerville (The Electric Arc) have more on steady state arcs.








> So It appears that the arc with the rod has higher current
> than just a plain arc.  The arc from the toroid to the rod
> seems dimmest of all.  The rod seems to be picking up some