or canvas, with a vent provided in the top for the excape of the moisture-laden air. The warm air must be adequately circulated over all surfaces to be dried, and if natural draft is not sufficient to accomplish this, small fans should be used to force circulation. machines may be conveniently dried out by heating in an oven. Small Regardless of method used, the temperature should not be permitted to exceed 90°C, The time required for thorough drying out will depend to a considerable extent on the size and voltage of the machine, and on the method of heating. Insulation resistance measurements should be taken at intervals of several hours, until the readings become fairly constant. The normal insulation resistance value as established by AIEE Standards is as follows: Megohms = Rated Volta f RatinginKVA + 1000 100 In event the KVA rating is not known, the Hp rating may be used to arrive at a sufficiently accurate value for practical purposes. Dry machines in good condition will have an insulation resistance value considerably higher than that obtained from the formula. A megohmeter offers tne simplest method of measuring this resistance as it gives a direct reading, but the voltmeter method can be used providing a proper direct current source is available. DIELECTRIC TESTS After the windings have been thoroughly dried out, and insulation resistance readings are satisfactory, a high potential test may be applied to determine the dielectric strength of the insulation. New windings should successfully withstand a high potential test of twice 3-52