The Kosterlitz–Thouless transition can be observed experimentally in systems like 2D Josephson junction arrays by taking current and voltage (I-V) measurements. Above T c , the relation will be linear V ∼ ∼ I . Just below T c , the relation will be V ∼ ∼ I 3 , as the number of free vortices will go as I 2 . This jump from linear dependence is indicative of a Kosterlitz–Thouless transition and may be used to determine T c . This approach was used in Resnick et al. to confirm the Kosterlitz–Thouless transition in proximity-coupled Josephson junction arrays.