For measuring low resistances using the Kelvin double bridge arrangement it was mentioned in our electrical measurement lab class that we should use both a forward current and a reverse current (i.e. in opposite direction) during any Kelvin bridge experiment. The final observation is made by taking the "average" of observations when using forward currents and reverse currents. Our teacher said that "thermal emf" generated when using a forward current would be opposite direction to the "thermal emf" generated when we use a reverse current. I'm not sure what "thermal emf" means in this context. Also could someone explain what exactly is the use of taking measurements using both forward and reverse currents?