Carnot established in 1824 that the efficiency $\eta_{ C}$ of reversible engines operating between a hot bath at absolute temperature $T_{ hot}$ and a cold bath at temperature $T_{ cold}$ is equal to $1-T_{ cold}/T_{ hot}$. Carnot particularly considered air as a working fluid and small bath-temperature differences. Plugging into Carnot's expression modern experimental values, exact agreement with modern Thermodynamics is found. However, in a recently published paper ["Sadi Carnot on Carnot's theorem", \textit{ Am. J. Phys.} \textbf{ 70}(1), 42-47, 2002], Guemez and others consider a "modified cycle" involving two isobars that they mistakenly attribute to Carnot. They calculate an efficiency considerably lower than $\eta_{ C}$ and suggest that Carnot made compensating errors. Our contention is that the Carnot theory is, to the contrary, perfectly accurate.
Categories: physics.hist-ph cond-mat.stat-mech