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Re: A Simple Lab Demo of Resistor Noise EMF.



At 10:14 3/20/01 -0500, Michael Edmiston wrote:

.... measuring very small rms values is
tough. One source (URL given below) indicates the meter can give an
internal noise reading of several digits if you are trying to measure an RMS
level that is 1% of the range you have selected. This might be some of what
people are seeing when they torch resistors.

Here is the URL for a site that discusses some of this. Perhaps you will
find it helpful: http://www.tmworld.com/articles/TME/08_1999_RMS.htm

/snip/
Michael D. Edmiston, Ph.D.

There's no doubt, it's easy to disillusioned about true rms meters
that are untrue, and ac milliameters that indicate totally illusory
numbers when hooked to resistors of reasonably high values.

For example, when I seek to restore my confidence in the omnipresence
of electrical noise by hooking a 6.2 volt 1N4735 zener diode to a
9 volt alkaline battery via a 1 megohm resistor, the $50 oscilloscope
has no trouble portraying a 10 millivolt envelope of the authentic n signal
sitting on the 5.82 volt DC from the zener. But the DVM labors under
the misapprehension that the noise value is a hundred times bigger
(again...)

Diodes are known to generate hundreds of times the thermal noise signal
predicted by the usual formula.
I wonder what sort of noise a Nodon Valve would produce?
This forgotten antique mentioned in the same WWII electronics dictionary
that describes the hot wire ammeter, says it is an electrolytic rectifier
consisting of an aluminum cathode rod in a lead [anode] container filled
with ammonium phosphate. It would offer a limited bandwidth, I imagine.

brian whatcott <inet@intellisys.net> Altus OK
Eureka!