Let’s see this in operation. Here is how we make
a Johnson noise thermometer. Essentially all we have to
do is connected any resistance sensor to a true RMS voltmeter.
The material of the sensor is unimportant. The resistor
is show in blue to indicate that it is cold but the meter
does not read zero sine it is not an absolute zero and Is
therefore generating some Johnson noise. The Johnson noise
is white, which means that it has the same power density
at all frequencies and our voltmeter will respond to frequencies
in its operating bandwidth. As we heat the resistor the
meter will indicate progressively higher readings as the
electrons that within the resistor are excited into high
energy states. The system is measuring the electrical noise
generate by the thermally excited electrons in the resistor
and as they get hotter their average speed increases thereby
increasing the noise generated. The noise is directly related
to the temperature of the electrons generating the noise.
