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[TCML] Black printing can be *very* conductive!

    Well, I was sitting in my office today, and I  recalled our discussion 
about the conductivity of black printing on  Sonotube cardboard forms.
    So I went to one corner, and picked up a  corrugated cardboard box with 
black printing on it and in the other corner  I got my megger.
    My megohmmeter is a slightly older AEMC Model 1015,  but it's been just 
fine for my purposes. I used the 1000V setting.
    The corrugated cardboard box had black printing on  the side. The part I 
tested had a black rectangle, 1" by 3", with an "unprinted"  5-digit number in 
the middle. Call it 60-70% black coverage in that  rectangle. And not the 
greatest printing job, as it wasn't as solid a color  as it could've been. 
    As I had posted earlier, I knew the black pigment  in the ink was 
basically soot, but I was astounded at how much a difference in  conductivity it made!
    From one end of the rectangle to the other, I read  1 Meg of resistance. 
Enough that the 1mA of current sparked form the probe to  the surface of the 
box, and I could instantly, and strongly, smell burnt paper!  If I touched the 
probe down just a coupla millimeters to the side of the black  rectangle, the 
resistance read infinite - or at least over 1000 Megs, according  to my 
megger. Interestingly, the smallest reading I could attain was 300K, with  the 
probes 1/4" apart on the black rectangle. So it doesn't seem linear with  distance.
    I was surprised that the basic cardboard box itself  was "infinite" 
resistance - the corrugated layers are held together with  water-based starch 
adhesive, and the paper layers themselves have some  moisture content.
    So there you have it! Black printing on cardboard  can be *dangerously* 
conductive - enough to pass a few mA at a few thousand  volts, or enough to 
ignite the underlying cardboard! And this was a rather weak  print job.
-Phil LaBudde
Center for the Advanced Study of Ballistic  Improbabilities

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