Chronology Current Month Current Thread Current Date
[Year List] [Month List (current year)] [Date Index] [Thread Index] [Thread Prev] [Thread Next] [Date Prev] [Date Next]

Re: A Simple Lab Demo of Resistor Noise EMF.



-----Original Message-----
From: Of Michael Edmiston
Sent: Monday, March 19, 2001 8:52 AM
Subject: Re: A Simple Lab Demo of Resistor Noise EMF.

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.

Carbon is a very complex material and at low heat treatment temperatures
somewhat inhomogeneous. Its electrical properties are a strong function of
its fabrication (heat treatment) temperature. It is possible that heating
it with a torch heated it to a higher temperature than it was fabricated at.
In addition, carbon starts to oxidize at about 600 C, I think. It is likely
then, that the carbon was changed by the torching. Heating it with a torch
heats it non-uniformly, leading to steep temperature gradients which can
cause thermoelectric voltages. Nonuniformities in composition can also lead
to thermoelectric effects during torching. The heating will increase the
carrier concentration probably more than it will increase scattering. By
the way, graphite itself is electrically quite a complex material - even
along the graphite planes - and its conductivity vs. temperature curve
changes from semiconductor-like to metallic-like as the heat treatment
temperature is increased from about 2200C to above 3000C. This is due to a
change in crystallite size only, which reduces the amount of scattering at
grain boundaries. Graphite is a semi-metal - there is a slight overlap of
conduction and valence bands.

Larry