The World's first practical Johnson Noise Thermometer
developed by .......METROSOL Limited

Johnson noise thermometry (JNT) is a primary temperature measurement technique based on the fundamental properties of thermal fluctuations in conductors.

A Johnson noise thermometer never needs calibrating and is insensitive to the condition of the sensor material, so is ideally suited to long-term temperature measurements in harsh environments (such as nuclear reactor coolant circuits, nuclear waste management and storage) or where low drift is required (such as high temperature reference standards in metrology or the annealing of single crystal turbine blades in the aerospace industry).

Johnson Noise Probe


At present all thermometers in use (thermocouples, platinum resistance thermometers, thermocouples) are secondary thermometers and are therefore prone to drift. They don’t actually measure temperature directly, instead a property of the sensor (such as resistance or EMF) is measured. The property is related to temperature by a calibration process. Either the generic relationship between the property and temperature is used or an individual calibration can be made if greater accuracy is required. In use, this relationship is employed to convert the measurand back to temperature, but this assumes that the relationship has not changed. In practice, this relationship between the property and temperature can change with time leading to “drift” in the reported temperature. This is particularly so in harsh environments where factors such as contamination of the sensor, changes in its physical structure, strain or transmutation (in nuclear environments) affect the property measured independent of its temperature.

Primary thermometers are quite different. With a primary thermometer, all the required properties of the sensor are measured and then fed into a fundamental physical law from which true thermodynamic temperature can be calculated.

Because the measurement of temperature is based on a fundamental physical law, primary thermometers do not (cannot) drift. Of course, the electronics used to measure the required properties can drift, but modern electronics are more than adequate to ensure that such drift is insignificant. In current practical thermometers, it is always the sensor rather that the electronics that give rise to the drift.



Johnson noise (thermal noise, Johnson-Nyquist noise, white noise) is the electronic noise generated by the thermal agitation of the charge carriers (usually the electrons) inside an electrical conductor, which happens regardless of any applied voltage. The generic, statistical physical derivation of this noise is called the fluctuation-dissipation theorem.


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 Johnson Noise