In modern notation we would write, I = E r + R , where E is the open-circuit emf of the thermocouple, r is the internal resistance of the thermocouple and R is the resistance of the test wire. In terms of the length of the wire this becomes, I = E r + R ℓ ℓ , where R is the resistance of the test wire per unit length. Thus, Ohm's coefficients are, a = E R , b = r R .