The fluctuation dissipation theorem

In 1928, J. B. Johnson showed experimentally that a resistor with no current flowing in it has a measurable electrical noise across its terminals. The signal of the magnitude of a few microvolts is called noise because when made audible through hi-fi audio amplifier and speaker, it has a high-pitched hissing quality. Noise from a current-free metallic resistor is called Johnson noise, or sometimes thermal noise, because Johnson identified the voltage fluctuations across the resistor with thermal agitation of the charge carriers. His classic experiments showed that the mean-square-voltage noise signal was directly proportional to the resistance and the absolute temperature in various types of solid as well as in liquid resistors.