This equation may be used to determine the correlation function ⟨ U ( t ) U ( t ′ ) ⟩ = k B T C exp ⁡ ⁡ ( − − | t − − t ′ | R C ) ≈ ≈ 2 R k B T δ δ ( t − − t ′ ) , which becomes white noise (Johnson noise) when the capacitance C becomes negligibly small.