# How to measure the Thermal Noise in a conductor?

Johnson Noise in a conductor is very small: between 10-9 to 10^−12 V

How to measure it using an oscilloscope and amplifier? If it were a resistor that would be easy, but conductors have much smaller voltages.

To start with: what is the smallest voltage an oscilliscope can mesure?

"Very small signal allows you to adjust the front end gain But can sometimes be overwhelmed by the inherent noise of the oscilloscope" - source: http://www.rohde-schwarz-usa.com/rs/rohdeschwarz/images/20150310-HD-Webinar-Presentation.pdf Measuring Low Voltage Signals Using an Oscilloscope March 10, 2015 Richard Markley Oscilloscope Product Manager

• I think that oscilloscopes are generally limited to signals greater than 1 mV or so because of Johnson noise limitations. Offhand don't know of an easy way of accurately measuring the level of Johnson noise in a resistor. Not just a simple matter of amplifying the noise with an amplifier and looking at it with an oscilloscope because the amplifier and oscilloscope will add their own Johnson noises to the original noise. – Samuel Weir Mar 25 '18 at 21:28
• @SamuelWeir For a resistor the JN in the oscilliscope will be small compared to that of the resistor being measured. Many undergrad labs use osilliscopes to measure these things - I personally did it with nothing more than an oscilliscope, resistor, and coaxial cables see: physics.usyd.edu.au/~senior-lab/3YL/Expt_5.pdf – Dale Mar 25 '18 at 21:33
• @SamuelWeir It seems you are correct: physics.usyd.edu.au/~senior-lab/3YL/Intro%20Talk.pptx – Dale Mar 25 '18 at 21:43
• Johnsons original paper used a vacuum tube amplifier and thermocouple. – Dale Mar 25 '18 at 23:10