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.