Change of specific electrical conductivity
Once there are free charge carriers in the liquid, they will cause an electric current equivalent to the existing potential. The fluid functions as an ohmic resistor, the output is effective power. Generally, all lubricating oils are characterized by a comparatively low degree of electric conductivity, so that they are normally considered to be non-conductors.
As the molecular structure of the fresh lubricant in combustion engines changes depending on the duration and the mode of application and the resulting wear effects, the electric conductivity of the lubricant also changes. However, this change is of a very limited extent so that a highly sensitive and precise measuring device is needed.