# Re: Displacement Current Revisited

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Bryan -

My understandung of displacement currents is that they refer to current
density which is not the same as the currents we are measuring with an
ammeter. To my knowledge no one has ever measured a displacement current or
even suggested how to measure these currents around a Tesla coil. How about
a fluorescent lamp? Apparently displacement currents are only one possible
method to describe electromagnetic fields using a density or unit volume as
a way to describe a quantity of electrical charges moving thru space.

I don't think "the equation implies phase shifted Id coaxial to I(real)"
because the Id is in the form of current density and the I(real) is in the
form of current intensity. The standard ammeter measures current intensity.
How would you measure current density around a TC except by knowing the
volume of a container such as a fluorescent lamp?

John Couture

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At 02:51 PM 3/20/99 -0700, you wrote:
>
>John,
>
>Personally, I find this discussion very relevant; but then again
>I'm not the EE type this list seems well populated by.
>
>I was actually looking for the Id equation, myself; Thank You
>
>If I am applying the concept correctly, (no guarantee of that)
>the equation implies phase shifted Id coaxial to I(real).
>
>However, dE/dt is max as E crosses zero.
>So I and Id are coherent.
>However this applies only to the inductor,
>in the situation of the TC toroid there must be (IMHO)
>a longitudinal Id to ground plane as well as a radial
>Id from space charge.
>
>But the only one that would
>conceivably not have a coherent I(real)
>would be the last one.
>Does anybody know if this is where
>free air streamers arise from?
>Or am I just oversimplifying?
>
>Bryan Kaufman
>
>Tesla List wrote:
>
>> Original Poster: "John H. Couture" <COUTUREJH-at-worldnet.att-dot-net>
>>
>>   Greg, Terry, All -
>>
>>   Nothing apparently has happened to the notion that "displacement" currents
>> are still considered to be caused by the changing E field. Cook gives the
>> equation:
>>
>>     Id = (eo)(dE/dt)
>>
>>   Id = Displacement current density
>>   (eo)(dE/dt) = per Maxwell's equations
>>
-------------------------   snip

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