Thermal Movement of Electricity in Conductors

Ordinary electrical conductors are sources of random voltage fluctuations that can be measured using sensitive enough devices. This property of conductors appears to be the result of thermal agitation of electric charges in the conducting material.

Observed Effect and Measurement

This effect has been observed and measured for various conductors, in the form of resistance units, using a vacuum tube amplifier ending with a thermocouple. This effect appears as part of the phenomenon known as “tube noise”. The emitted portion from the resistance leads to a root mean square voltage fluctuation V2 that is proportional to the value R of that resistance. The ratio V2 / R is independent of the nature or shape of the conductor, being the same for metal wire resistors, graphite, thin metal films, ink films, and strong or weak electrolytes. However, it does depend on temperature and is proportional to the absolute temperature of the resistance. This temperature dependency indicates that the noise component proportional to R comes from the conductor and not from the vacuum tube.

Edit Summary

While attempting to improve the design and manufacture of vacuum tubes, pioneers in electronic engineering discovered a fundamental problem – noise. Walter Schottky first proposed the existence of thermal noise and shot noise in 1918. In a letter to the journal Nature in 1927, J. B. Johnson commented on the voltage fluctuations that appear “as a result of the thermal agitation of electric charges in the conducting material.” Johnson would later become associated with thermal noise – now also known as Johnson noise – after publishing a definitive experiment on noise in 1928, along with a theoretical explanation by Harry Nyquist [Johnson, J. B. Phys. Rev. 32, 97-109 (1928); Nyquist, H. Phys. Rev. 32, 110-113 (1928)]. However, his 1927 letter aimed to bring “a similar phenomenon” to the attention of Nature readers. In this instance, the fluctuations depend not on temperature but inversely on frequency – Johnson discovered “1 / f noise.”

Source: https://www.nature.com/articles/119050c0