Re: 2nd law of thermodynamics in question

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.

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.

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.

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.

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.


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.

--
Richard Herring
.

Relevant Pages

  • Re: 2nd law of thermodynamics in question
    ... T-connector, leaving the scope probe connected to it. ... The other scope connector is some distance away from ... then placing the ground lead where the + lead was located. ... At such a point the pulse encounters two wires. ...
    (sci.physics)
  • Re: 2nd law of thermodynamics in question
    ... a pulse contains no _directional pulses_ and such a signal merely ... know the direction of energy flow. ... The other scope connector is some distance away from ... then placing the ground lead where the + lead was located. ...
    (sci.physics)
  • Re: An OT oscilloscope question
    ... don't want current flowing in the ground lead or you will burn up ... doing you should not float the scope or alternator and obviously ... Next is to use a 10x or 100x probe and connect it to the alternator ... This will allow you to check one leg. ...
    (rec.crafts.metalworking)
  • Re: hot scope leads?
    ... in a simple 12AX7 into a 6V6 SE design guitar amp. ... Next thing I know I can smell buring and the ground lead of the probe ... I turn everything off and barely salvage the scope lead (the ground ...
    (rec.audio.tubes)
  • Re: Tech: WPC Resets
    ... I ascribed it to loose chips (of course it ... Did you check the ground on the CPU board with your scope? ... trying to track down a reset problem on my TZ with a scope and noticed ... I made a bandaid fix intead of tracking down the bad connector. ...
    (rec.games.pinball)