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Re: A Simple Lab Demo of Resistor Noise EMF.



I did a fair amount of work with "noise" in my former life as a nuclear
detector builder.

Other responders to this thread are correct that the torched resistor hooked
to a simple AC voltmeter cannot be demonstrating resistor noise.

Thermal AC noise in a resistor is also called Johnson noise (J.B. Johnson
did a lot of experimental studies) or Nyquist noise (Harry Nyquist worked
out the math).

Thermal, Johnson, Nyquist noise is V(rms) = sqrt(4kTRB) where k is
Boltzman's constant, T is Kelvin temperature, R is resistance, B is
bandwidth). This is 1.27 microvolts for a room-temperature 10kohm resistor
measured with a bandwidth of 10kHz.

Since the temperature dependence appears under the square root, going from
300 K to 1200 K would only double the rms voltage up to 2.6 microvolts, and
1200 K is way hotter than a red-hot resistor. We cannot get into the
millivolt range by torching the resistor.

A fairly good multimeter ($50-$75) has an AC bandwidth of 10kHz, and it
falls off pretty rapidly after that. We cannot get into the millivolt range
by increasing the bandwidth.

Switching from a 10kohm resistor to a 1Mohm resistor would be a 100-fold
resistance increase, and that would increase the Johnson-Nyquist noise by a
factor of ten (compared to a 10kohm resistor), giving us about 18 microvolts
for a 1Mohm resistor heated to about 600 K with a meter having a bandwidth
of 10kHz.

Therefore... if signals of millivolts are being seen when torching a
several-hundred-kohm resistor, this cannot be thermal noise. As other have
stated, it might be junction potentials caused by different temperatures at
the junctions (i.e. a thermocouple, i.e. Seebeck effect).


Michael D. Edmiston, Ph.D. Phone/voice-mail: 419-358-3270
Professor of Chemistry & Physics FAX: 419-358-3323
Chairman, Science Department E-Mail edmiston@bluffton.edu
Bluffton College
280 West College Avenue
Bluffton, OH 45817