The factors governing the temperature-rise of electrical machines are so interwoven that the only satisfactory method of separating them is to test actual machines with different distributions of losses, different speeds, etc.; conclusions derived form tests on models must be accepted with caution. In this paper are given the results obtained on two electrical machines, one with a rotating armature and the other with a rotating field, which were tested with different load and field currents, different speeds, etc. The machines were also tested when totally enclosed and when cooled by an induced draught. Temperature-rises were measured by thermometer, by thermo-junction and by the increase of resistance. In the case of the machine with the rotating armature it was found possible to follow the temperature variation of the latter by placing a thermo-junction a short distance away from the armature, and it si suggested that this method might be used for shortening the duration of heat-runs.