In physics and chemistry, the Nernst effect (also termed first Nernst–Ettingshausen effect, after Walther Nernst and Albert von Ettingshausen) is a thermoelectric (or thermomagnetic) phenomenon observed when a sample allowing electrical conduction is subjected to a magnetic field and a temperature gradient normal (perpendicular) to each other. An electric field will be induced normal to both.

This effect is quantified by the Nernst coefficient ${\displaystyle \nu }$, which is defined to be

${\displaystyle \nu ={\frac {E_{y}}{B_{z}}}{\frac {1}{\partial _{x}T}}}$

where ${\displaystyle E_{y}}$ is the y-component of the electric field that results from the magnetic field's z-component ${\displaystyle B_{z}}$ and the x-component of the temperature gradient ${\displaystyle \partial _{x}T}$.

The reverse process is known as the Ettingshausen effect and also as the second Nernst–Ettingshausen effect.