The second law rules out a perpetual motion machine of the second kind.
Such a machine would cyclically convert heat taken from a single thermal reservoir entirely into work, with no other effect. The first law would not necessarily forbid that operation, because energy could still be conserved. The second law does forbid it because the process would reduce the entropy of the overall situation without compensation elsewhere.
The first law instead rules out a perpetual motion machine of the first kind, which would produce work without an energy input. This distinction is a common source of confusion. The “second kind” does not refer to the second law’s historical numbering alone; it describes a machine that violates the second law’s restrictions on heat engines.
The limitation emerged from nineteenth-century heat-engine theory. Sadi Carnot studied engine efficiency in 1824, while Rudolf Clausius and William Thomson developed formal second-law statements. A practical heat engine must operate between reservoirs at different temperatures and discharge some waste heat, so its efficiency cannot reach 100 percent.