In reply to Stephen A. Lawrence's message of Thu, 19 Jan 2006 08:35:01 -0500: Hi, [snip]
All op-amp's have a leakage current. How much resistance do you measure with one of the alligator clips disconnected? >After reading Bill Beaty's comments, I took another stab at it, and >actually got a result! > >With a #6B artist's pencil I drew (free-hand) two dark solid lines on a >sheet of printer paper, about 200 mm long and about 2.5 mm apart. I >checked them, and each line was electrically "intact" (#6 pencil marks >have a tendency to leave electrical gaps if one isn't careful). I >connected to the marks with alligator clips -- which were verified to >make decent contact in these conditions -- and put the paper in series >with a 10M resistor (paralleled with a 100pf capacitor), with 9 volts >across the lot. > >I read out the voltage on the resistor through an op-amp, amplified >1000x, and got a reading of about 3.5 volts, which wandered around a >lot. Breathing on the paper sent the reading off-scale; it recovered >slowly. Laying a thumb across the marks shot it off-scale as well. > >3.5 volts works out to a resistance of about 27 gigohms. > >Dividing that by the width of the strip and multiplying by the length, >that comes out to a (surface) resistivity of about 2000 gigohms for 20 >pound 100% recycled ink-jet paper. > >At some point I'll try it again with somewhat better-controlled >conditions, including trying to get connections to _both_ sides of the >paper rather than just one side -- though that shouldn't actually make >much difference. Regards, Robin van Spaandonk http://users.bigpond.net.au/rvanspaa/ Competition provides the motivation, Cooperation provides the means.
