yoshinaga@cerritos.edu (Michael Yoshinaga) (07/24/90)
> Here's the real scoop (source: The Feynman Lectures on Physics, Vol 2, > chapter 9). Away from thunderclouds, the earth has a negative charge > relative to "the sky." The potential difference is quite incredible: > about 100 volts per meter. Why does this not electrocute you? Because > your skin is so much better at conducting than air, your skin can be > treated as a perfect conductor. Your head and your feet have the same > potential as the earth. A foot from your nose, the air is at a potential > of +200V. > > richard@locus.com Just curious... Why can't this be measured with a DVM? Thanks, Mike -- AM: michael yoshinaga EM: yoshinaga@cerritos.edu VM: (h)714-761-4322 SM: 11110 e. alondra blvd; norwalk, ca. 90650 Disclaimer ?: Admit nothing, deny everything, demand proof
cage@fmeed1.UUCP (Russ Cage) (07/24/90)
In article <5498.26aba906@cerritos.edu> yoshinaga@cerritos.edu (Michael Yoshinaga) writes: >> chapter 9). Away from thunderclouds, the earth has a negative charge >> relative to "the sky." The potential difference is quite incredible: >> about 100 volts per meter. > >Why can't this be measured with a DVM? Because the source impedance is incredibly high; a 100K ohm/V DVM won't see enough current to move it. Using better contacts (radioactives, which ionize the air slightly and make it conduct, bringing the contact to the same potential as the air for small but reasonable currents), you can measure this gradient quite simply. Wing-leveler autopilots for model airplanes have been made with a couple of such contacts and some op amps. -- Russ Cage Ford Powertrain Engineering Development Department Work: itivax.iti.org!cfctech!fmeed1!cage (Business only) Home: russ@m-net.ann-arbor.mi.us (Everything else) I speak for the companies I own, not for the ones I don't.
jeffw@midas.WR.TEK.COM (Jeff Winslow) (07/25/90)
|> Here's the real scoop (source: The Feynman Lectures on Physics, Vol 2, |> chapter 9). Away from thunderclouds, the earth has a negative charge |> relative to "the sky." The potential difference is quite incredible: |> about 100 volts per meter. Why does this not electrocute you? Because |> your skin is so much better at conducting than air, your skin can be |> treated as a perfect conductor. Your head and your feet have the same |> potential as the earth. A foot from your nose, the air is at a potential |> of +200V. | |Just curious... |Why can't this be measured with a DVM? Because that 10 megohm input DVM is also a much better conductor than air. Maybe with something like an electrometer... ?? Jeff Winslow
bobt@pogo.WV.TEK.COM (Bob Tidrick) (07/25/90)
>> about 100 volts per meter. Why does this not electrocute you? Because >> your skin is so much better at conducting than air, your skin can be >> treated as a perfect conductor. Your head and your feet have the same >> potential as the earth. A foot from your nose, the air is at a potential >> of +200V. >Why can't this be measured with a DVM? This potential has an "extremely" high impedance. A static meter just might measure it. -- Bob Tidrick GPID Engineering Tektronix Inc. Wilsonville OR.
tomb@hplsla.HP.COM (Tom Bruhns) (07/25/90)
>|> potential as the earth. A foot from your nose, the air is at a potential >|> of +200V. >| >|Just curious... >|Why can't this be measured with a DVM? >Because that 10 megohm input DVM is also a much better conductor than air. >Maybe with something like an electrometer... ?? I believe the folk who measure this (e.g., atmospheric physicists) use an instrument called a "field mill." A prof whose office was next to mine used to build them and drop them through thunder storms.
kenny@m.cs.uiuc.edu (07/26/90)
...A foot from your nose, the air is at a potential of +200V. ..Just curious... ..Why can't this be measured with a DVM? .Because that 10 megohm input DVM is also a much better conductor than air. .Maybe with something like an electrometer... ?? Yup. There's an article in an old Amateur Scientist column (back in the Stong era) about this. The circuit presented is ridiculous by today's standards -- lots and lots of vacuum tubes. I bet someone could do a real nice job with an instrumentation-grade FET-input opamp. Have to be really careful about common-mode noise elimination. Also would have to bleed the charge off the antenna periodically. I may design such a beast eventually; a measure of atmospheric charge is a valuable datum for short-term weather forecasting. Kevin, KE9TV kenny@cs.uiuc.edu ke9tv@uiuc.edu
irwin@m.cs.uiuc.edu (07/26/90)
/* Written 2:29 pm Jul 25, 1990 by kenny@m.cs.uiuc.edu in m.cs.uiuc.edu:sci.electronics */ >...A foot from your nose, the air is at a potential of +200V. >..Just curious... >..Why can't this be measured with a DVM? >.Because that 10 megohm input DVM is also a much better conductor than air. >.Maybe with something like an electrometer... ?? >Yup. There's an article in an old Amateur Scientist column (back in >the Stong era) about this. The circuit presented is ridiculous by >today's standards -- lots and lots of vacuum tubes. I bet someone >could do a real nice job with an instrumentation-grade FET-input >opamp. Have to be really careful about common-mode noise elimination. >Also would have to bleed the charge off the antenna periodically. I >may design such a beast eventually; a measure of atmospheric charge is >a valuable datum for short-term weather forecasting. >Kevin, KE9TV >kenny@cs.uiuc.edu >ke9tv@uiuc.edu /* End of text from m.cs.uiuc.edu:sci.electronics */ I hobby in radio contolled model aircraft, and recall that about 15 years ago, a Maynard Hll from JPL labs, who also flies R/C models, built an electrostatic auto pilot. He used op amps, the sense elements on the wing tips tied to the op amps was elements removed from a draftman's brush, obtained at a local drafting supply store. I do not recall what the material was. The op amp output was tied into the servos such that the R/C system could over ride the auto pilot, but the autopilot would maintain straight and level flight, by measuring the voltage in the air at the wing tips. He also had one on the tail of the aircraft. This is the same Maynard Hill that flew models of the 747 and shuttle at JPL labs to test the piggy back set up in model form, before they tried it full scale. There are video tapes available that our model club has checked out for viewing, showing this testing. Al Irwin irwin@cs.uiuc.edu