Of lightning strikes and measuring...

To: mail11: ;
Subject: Of lightning strikes and measuring...
From: THIS SPACE LEFT BLANK <pierson-at-msd26.enet.dec-dot-com>
Date: Wed, 19 Jun 96 19:46:44 EDT
Cc: pierson-at-msd26.enet.dec-dot-com

For HF discharges (lighining and Tesla) Long 'ground' wires tend to
have such a high Xl as to not react as expected.  Sparks commonly take longer
paths to shorter grounds.

=========
Random thoughts on mesuring currents:

TC milliameters are good, tru rms meters with a few caveats:
	MANY times the thermocouple/heater is EXTERNAL.  If the meter face says
	thermocouple-type it may well ned an external TC/heater to function.

	The heaters can be delicate, subject to overload, blown and they are
	gone.

On the other hand, such a meter is 'true rms' (as has been noted) which can
be good.  One less than ideal thing, for trying to read the current twixt
resonator and extra coil is that the TC leads themselves are conductors.
Fine and dandy for base currents in a grounded operation, but not real
easy to work with, as the 'top' of a coil is at 'lotsa' volts.

Blueskying some ideas to read currents at an elevated voltage:

	homebrew a tc/heater, hard wire to a 'mirror-galv'.  Light source
	either local battery or an externally mounted beam.  Mirror galv
	can be home brewed, too.  Light beam, can be read remotely, tho
	might need some light blocking tubes.

	Variant:
	as the currents are probably 'non zero', get some fast (schottky)
	diodes, across a shunt and stick in a conventional meter.  Read direct
	with binocular/telescope (as suggested elsewhere) or cement mirror
	to needle and project the light beam.  Probaly wants an RF tight box.

	Variant:
	battery powered amp (rf proof box, again) driving meter or LED.
	Now we get to some fun stuff: coupel the LED to a fiber optic, and
	read remotely at other end.  Distances are relatively short (10s of
	feet) so fancy expensive fiber is not needed.)

	Variant:
	Back towards 'low tech'.  stick a low voltage light bulb in
	series with the 'feed line', proably with a shunt.  Capture light to
	optical fiber as above.  Notable advantages: cheap.  rugged.  Might
	could use LED's. i'll pass to more experienced heads on that.

Another option, towards the simple end, is the 'hot wire' instrument.  The
current heats a wire, which stretches/sags, and this mechanical motion moves
a pointer.  (I have found these hiding in auto dashboards as voltmeters and
gas guage indicators.)  With care can be very flat with respect to freq.  Again
a 'mirror galv' approach could be used, or a telescope....  Can be made very
rugged.

All of these would need to be calibrated, esp. the last few.  Could do exotica, 
like duty cycle coding the led output for current, but i suspect thats overkill.

(I can expand on any of these if they seem practical or of intrest.  Or i can
go back to lurk mode...)

========================
Currents in a closed, dc, or LF AC system are the same thruout a circuit.
Currents in a physical resonator are not.  Just as there is a 'voltage peak'
at the top of a tesla coil, there is a current peak at the bottom.  (assuming
the usual geometry....).  I'd not hazard a guess at the currents at the junction
between a 'tesla secondary' and its tertiary, in part because i have heard
different definitions of what the tertiary IS.  And all the guesswork is
of no point to one well made measurement...

=================================
20-20 hindsight department:

Power companies routinely put thermometers, sometimes remote indicating
data acquisition gear, on large, expensive, pieces of power handling gear,
notably capacitors and transformers.  Based on the experience reported here,
if i had a $200 capacitor or transformer, i'd stick a $2 thermometer on the
case.  Might not tell me anything, but then again...

	regards
	dwp