On Tue, 6 Feb 2018 14:32:31 -0500, "Jim Wilkins"
wrote:

"Larry Jaques" wrote in message
news
On Mon, 5 Feb 2018 20:09:08 -0500, "Jim Wilkins"
wrote:

"Larry Jaques" wrote in message
news On Sat, 03 Feb 2018 23:48:24 -0500, Clare Snyder

wrote:


Ah dinna know that. What was the actual resistance, or did you
measure it?

How would you measure a very low resistance? It isn't easy.

My assumption was that you knew what it was since you used it.
Isn't
that the way we do things? Find out everything we can about a
subject
then build what we can using the knowledge gained? It's less
expensive than trial-and-error, and Buddha knows I'm che...frugal.

So, how did clare know to use it? Similar to nichrome, but why was
nichrome used, etc? Copying other equipment without specs can lead
to
that kind of experimentation, but I was curious. About everything.
Coulda called me George, ah reckon.

And very low resistances are measured with very-low-resistance
meters,
of course. :-) I've never seen a Kelvin bridge and haven't had a
need
to measure anything with a super low resistance before. I read
about
them at Coleman decades ago. Voltage drop calculations are hard
with
tiny resistances, too. How would you do it, Jim?


I have more lab test gear than most people, so my exact methods may
not help others. Usually I force 1.000A and read the voltage drop in
milliVolts as milliOhms. This is good enough to find connections that
need cleaning in my TV antenna or solar panel wiring.

OK. Yeah, you probably do.


The hard part is forcing a stable constant DC current without special
equipment. If you use a battery the voltage keeps decreasing. Car
headlights works fairly well as the current limiter if you don't need
much accuracy. A battery, headlight and HF DVMs for current and
voltage would be enough to measure the motor speed control resistance
but not to make and calibrate a meter shunt.

What? Metrology labs don't use HF DVMs?

--
Stoop and you'll be stepped on;
stand tall and you'll be shot at.
-- Carlos A. Urbizo