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When designing a large steam-turbine-driven alternator the volume of air per minute needed to cool the machine is easily calculated if the losses are known, and the allowable air temperature rise is assumed. With data hitherto available it has not been possible to estimate the pressure needed to drive the air through the various paths, nor to predict the distribution of air in the machine. The problem is too complicated to admit of an analytical solution, and even an experimental investigation becomes involved. In general, a complete machine is not suitable for an experimental study of air flow, as changes in the series or parallel paths, the introduction of restrictions, etc., can be made only with difficulty, delays and expense. Accordingly, two models, to imitate two radial systems of ventilation, were built. Since ventilation, as such, was to be investigated, no electrical nor magnetic losses were introduced. The stator cores were made of wood, with suitable vent fingers of steel, and with blocks of wood to imitate coils in the slots. The rotor was the same for both models, and was made of steel, ventilated as in a usual machine, but with wood in the slots to imitate coils. Its diameter was such as to give the peripheral speed adopted in the largest turbo alternators; its length was about half of the probable largest rotor of that diameter. An external blower was used chiefly because then the delivered pressure could be independent of the rotor speed.