The device principle of an RTD is that resistance varies linearly with -------

The principle of an RTD, or Resistance Temperature Detector, is based on the property of metals that their electrical resistance changes in response to temperature variations. Specifically, for most materials used in RTDs, such as platinum, the resistance increases linearly as the temperature rises. This relationship allows for accurate and precise temperature measurements, as the change in resistance can be directly correlated to temperature changes.

The RTD operates on the fact that as the temperature increases, the atoms within the material vibrate more, leading to increased resistance to the flow of electric current. This linear relationship is fundamental to the functioning of the device, enabling it to be used effectively in a wide range of temperature sensing applications.