Re: TC Electrostatics


A Weston thermo-galvanometer was placed in series with the base of the 
TC helix and earth ground.  TC Imax is at the base.  The meter is a RF 
meter and the scale is 0-100 and represents I^2.  The meter pegs about 
0.25" on both sides of the 0 and 100. The meter is rated at R = 5.2 
ohms at I = 115 mA.  This was a $2 hamfest buy, but a real nice meter.  
I first fired the coil at low power with 8 and 5 Amp RF thermometers 
and got no reading.  Eventually I settled on the above meter.  I can 
adjust TC firing by adjusting neon primary voltage. I fired the TC at 
57 vac.  Various caps and diodes were place in shunt across the meter.  
The findings are tabulated. (The square root of I is not taken.)

Input 57 Volts

meter only  Shunt Cap   -->| to grnd    |<-- to grnd   Blocking diodes 

+91 ma       +4 ma     +5-10 ma irreg      +40 ma         +91 ma

There is no grossly observed increase or decrease in TC spark output. 

RF thermal meters are very interesting.  They essentially convert all 
current, including DC up to very high RF range, resistively into heat.  
They perform a perfect resistive integration of any wave form to rms 
energy in the form of heat.  This heat drives a thermocouple and hence 
the meter. The devices are very compact and usually have a very linear 
scale and fairly good response time.  These meters measure + & - DC and 
all AC currents as a positive deflection. 

All measurements were all performed by shunting the meter.  Back to 
back blocking diodes were exactly the same as the meter in series with 
the TC base.  Shunt diodes positive to grnd conducted more current 
(less current through meter) than diodes negative to grnd.  This 
difference in base current direction may be related to electrostatic 
emission from the top terminal.

Placing a thermometer in series with the base of a TC does not appear 
to affect its performance.  It automatically integrates total RF 
current in the TC base.  The meter is sensitive to TC current changes 
and a "rough" running coil makes current measurements a bit irregular.  
As the coil is tuned an runs smoother the meter reflects higher current 
and measures very smoothly.  This is a good instrument for experimental 
measurements and tuning a TC.  

I am currently conducting base "series experiments" using the same 
components.  I am also putting a larger current thermometer in series 
with the primary circuit, using a current shunt of course.