In this paper I describe the demon in macroscopic terms. Imagine “a being” that can follow the flow of heat and divert some of it to flow through a contrivance—a design, or machine—that produces power, mechanical or electrical, Fig. 1b. This happens everywhere in nature, from the whole earth as a heat engine, to every animal as a vehicle with its own motor, Fig. 1c.

Start with Fig. 2a. The box is filled with a gas of uniform temperature T 1 and pressure P 1 . The gas is moving in the box, with the kinetic energy KE 1 (state 1). Next, partition the box into A and B. The partition is highly conductive to the flow of heat. In one spot on the partition, the designer installed a sensitive instrument that measures the pressure on the two surfaces of the partition. Such a design can be built, operated, recorded and described.

Time varying pressure differences occur across the partition, at every point, because when jets and eddies hit the wall the fluid stagnates and experiences a pressure rise (the stagnation pressure). The instrument monitors the pressures on the A and B sides of the partition. Whenever the B side is at a higher pressure than the A side, the instrument opens an orifice through which B fluid flows into the A chamber. This process continues until all the motion stops. In that final state the isolated system is isothermal and the mass and pressure in A are greater than in B.

The thermodynamic analysis is detailed in the Supplement. In brief, the system consists of A and B and is isolated. State 1 is the initial state of uniform temperature T 1 , pressure P 1 and mass m. The reference state is (T 0 , P 0 ) and KE 1 is the kinetic energy of all the fluid in motion at state 1. State 2d is a constrained equilibrium state. The temperature is uniform (T 2 ), the partition is closed and the pressure on the A side (P 2 + ΔP) is greater than on the B side (P 2 − ΔP). The mass inventories of A and B are (m/2 + Δm) and (m/2 − Δm), respectively. The equilibrium pressure P 2 is in state 2 without partition. From PV = mRT for state 2 with and without partition, we find ΔP/P 2 = 2Δm/m. From PV = mRT for state 1 and state 2 (without partition) we find P 2 = T 2 P 1 /T 1 . The system is isolated, therefore its energy remains constant, regardless of whether the partition is present in state 2. The entropy inventories at states 1, 2 and 2d are derived in the Supplement: S 1 , S 2d and S 2 .

In particular, when ΔP/P 2 ≪ 1 the entropy change from state 1 to state 2d (with partition) is S 2d − S 1 = mc v ln(T 2 /T 1 ) − mR(ΔP/P 2 ). The second law requires S 2d − S 1 ≥ 0 and this means that the excess pressure (ΔP) that can be expected on the A side cannot exceed a value dictated by the initial kinetic energy present in the gas system, namely ΔP/P 1 ≤ KE 1 /(mRT 1 ) when T 2 − T 1 ≪ T 1 and ΔP ≪ P 2 . If after state 2d the partition is removed, the system reaches state 2 without partition (E 2 , S 2 ). The entropy change from sate 2d to state 2 is S 2 − S 2d = mR(ΔP/P 2 ) > 0.

The second law is obeyed by all the processes possible in this macroscopic version of Maxwell's demon, namely processes 1 → 2d, 2d → 2 and 1 – 2. The difference between the two scenarios is one of scale. In Maxwell's microscopic view, the designer and the instruments are so small and accurate that they can detect velocity differences between individual molecules. In the macroscopic scenario presented in this paper, the designer and the instruments are at a much larger, visible and palpable scale.

Key is the system feature that unites the two scenarios. The partition that opens and closes in accord with measurements of differences between the A and B sides represents design, or organization—a flow configuration with a purpose, or a function. The system without partition does not have design. The macroscopic scenario makes the design evident, much more visible than Maxwell's microscopic argument.

Design can be measured and its value is a characteristic of the system that possesses design. It is the ability of the system to generate useful energy (work, exergy, available work4). The value of the design in Fig. 2a is estimated by comparing state 2d with state 2. The system has the potential to produce work (useful energy, exergy) if placed in communication with one temperature reservoir, T 2 . This quantity is the exergy (available energy) at state 2d relative to state 2, namely , where U 2d = U 2 , as dictated by the first law and (T 2 , P 2 ) is the reference state, therefore (cf. Supplement) in the limit ΔP/P 2 ≪ 1,

The physical value of the design increases rapidly with the design's ability to achieve a pressure difference across the diathermal partition at the end of the process 1 → 2d. In the work producing process contemplated in the analysis that leads to Eq. (1), the useful energy produced is dissipated in a brake and the generated heat is absorbed by the system itself, which is at T 2 .

Interesting is that the physical value of this design, Eq. (1), is analogous to the physical value of Maxwell's design, Fig. 2b, where the system (m) is initially in state 1, at temperature T 1 and pressure P 1 . At state 2, Maxwell's system has design: two chambers, A and B, at different temperatures, T 1 + ΔT and T 1 − ΔT, separated by an adiabatic partition with an orifice opened and closed withpurpose. The design value is the exergy at state 2 relative to the reference (dead) state (T 1 , P 1 ), namely and (cf. Supplement)

This is analogous to Eq. (1). The value of Maxwell's design increases rapidly with its ability to build the temperature difference 2ΔT across the adiabatic partition by opening and closing the “smart” orifice.