John Bertrand Johnson was an electrical engineer whose 1928 measurements established the thermal electrical fluctuations now called Johnson-Nyquist noise. He detected random voltage fluctuations in conductors through a vacuum-tube amplifier and a thermocouple, and related the measured effect to temperature, electrical resistance and the measurement system's frequency response.[ref]
Thermal Agitation of Electricity in Conductors by John Bertrand Johnson. Publisher abstract, measured fluctuations and apparatus
This reveals why the average voltage alone is not enough to characterize a fluctuating signal. Its expected value can vanish while its mean-square value is positive. The fluctuations become an experimentally measurable noise floor in sensitive electronics, and their temperature dependence connects circuit measurements with the Boltzmann constant.[ref]
Thermal Agitation of Electricity in Conductors by John Bertrand Johnson. Abstract, determination of Boltzmann's constant and technical disturbance
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