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Since the insulation of windings in electric machines has comparatively low temperature limits, the problem of cooling these machines with the most economical use of material becomes one of major importance. The design of such machines from a temperature standpoint is usually based on tests of a previously made similar machine or else is of the ``cut arnd try'' type where such tests are not available. The predetermination of the operating temperature depends a great deal upon the rate at which the heat losses can be liberated from the ventilating surface to some cooling fluid such as air, which is considered in this paper. Some data are available regarding this rate of heat dissipation with forced air convection currents; a comparison of the various results published, however, shows them to be inconsistent. The purpose of this paper is to submit additional information that should be of value to the industry and that will also explain some of the inconsistencies in the past tests. Extensive experimental tests were made regarding the rate of surface heat transfer with air flows of various velocities for radial and axial ducts. The influence of shape, size, cross-section, condition of surface, mean temperature, and other factors were investigated. The most important factor found was that this rate of heat transfer was not constant along the air flow path but that its value varied from point to point along the duct.