Re: 2nd law of thermodynamics in question

Richard Herring wrote:
In message <1164127036.944959.158410@xxxxxxxxxxxxxxxxxxxxxxxxxxxx>, Paul
<softwarelabus@xxxxxxxxx> writes

Mind reading again? That's the first time I've asked that question. You
referred to exchanging the conductors. I'm asking about swivelling the
T-connector, leaving the scope probe connected to it. You didn't answer
that question.

The goal as I understand is an attempt to measure the current

The potential difference.
as close
to the T-connector.

_Across_ the T-connector.

That's fine, but how are you proposing to do that given the T-connector
splits?




I am referring to a two-connector common voltage
probe-- key word is "common." One of the scopes connectors is next to
the T-connector. The other scope connector is some distance away from
the T-connector, depending upon the scopes performance. As you know,
there must be two pinholes in coaxial cable for measurement purposes,
unfortunately.

Two? The T-connector constitutes one, I can't see another.

Of course. As stated, the method measures the pulse using the two lead
connectors of a voltage probe.




What I mean by swapping the voltage probe connectors is
literally placing the + lead where the ground lead was located, and
then placing the ground lead where the + lead was located.
Understandably, if the ground lead has precisely the same
characteristics as the + lead then such a technique fails. Fortunately
ground leads are connected to a common ground, thereby connected to an
appreciable amount of grounding system.

Which is capacitively coupled to other parts of the system under test,
so you're no longer measuring the potential at the single point you
thought you were.

If you want to measure the voltage pulses across a line then obviously
the lines ground cannot be part of the scopes grounding system.
Otherwise you'll short out the transmitter that's connected to the
coax. I usually work with two grounding systems. I have no problems
measuring incoming pulses. :-)




Traversing pulses are sensitive
to changes in electrical area.

To be more precise, they're sensitive to the characteristic impedance
of the transmission line. Connecting the high-capacitance ground lead
is changing it - and you have to consider what that capacitance is
coupling it _to_.

Consider a pulse traversing along a thin
wire. At such a point the pulse encounters two wires. The energy
contained in the pulse splits traversing along both wires.

And some of it is reflected.

Very true.



Essentially
such dispersion is what happens when the pulse encounters ground.
Relatively speaking, such an observed signal is distinct when compared
to repeating the pulse experiment and reversing the scope probe
connector leads.

Moreover, if you analyse where _all_ of your pulse goes, you may well
find that some of it is finding its way from the probe through the scope
to ground. Since no scope's common-mode rejection is perfect, that's
going to give you another spurious signal.

Very true.




Understandably such a technique is perhaps not the beautiful textbook
example you were expecting. You would perhaps give such a student an F
grade even though such a technique theoretically works. ;-)

He'd get an A if he can account for _all_ the paths the current takes
;-)

What he certainly isn't doing is measuring a single potential difference
between two well-defined points in the circuit.

What
makes such a technique undesirable is the fact it requires appreciable
amount of research to calibrate such a working system.

It's still not the _single_ measurement I was asking for. And it
doesn't work at all for a continuous signal. You've disappointed me.

Yet it still works given that I clarified such a method was for
measuring an incoming pulse. Lets not re-circulate that discussion. As
stated too many times already, there is no incoming pulse wave to
measure in a steady oscillating signal. How can you expect me to do the
impossible? Disappoint you??? I don't understand your position? I
clearly stated the method was for measuring a pulse.



Regards,
Paul

.

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