The authors have developed a very simple new test for measuring, as a separate quantity, the harmonic-frequency losses in squirrel-cage or slip-ring motors. The new test has the special merit that it uses only apparatus available in every routine-test department. Its principle is discussed first, and then certain operating points are considered.To establish the validity of the new test, several slip-ring machines were tested both by the new method and by two other existing methods. Of the three methods, only the new one can be applied to squirrel-cage motors; and, to illustrate this application, a summary of test results for a large number of medium-sized industrial squirrel-cage motors is given. Appendices on corresponding stator and rotor voltages, and on hysteresis drag, conclude the paper.