Two synthetic-temperature tests for use both on induction and synchronous machines have recently been described. The tests can be performed on free-standing machines with only their losses supplied from external electrical sources. The original test is simple to perform, particularly for induction machines, but it can result in abnormal secondary losses when applied to machines which employ skin-effect rotors. A new test which completely eliminates this imperfection has been devised. The test embodies the principles of both the earlier tests.