There is a close analogy between the paradigmatic Brownian particle discussed above and Johnson noise, the electric voltage generated by thermal fluctuations in a resistor. The diagram at the right shows an electric circuit consisting of a resistance R and a capacitance C. The slow variable is the voltage U between the ends of the resistor. The Hamiltonian reads H = E / k B T = C U 2 / ( 2 k B T ) , and the Langevin equation becomes d U d t = − − U R C + η η ( t ) , ⟨ η η ( t ) η η ( t ′ ) ⟩ = 2 k B T R C 2 δ δ ( t − − t ′ ) .