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Data are presented from comparison of nondestructive and destructive tests on several synchronous generator stators ranging from 800 to 33,000 kva. It is shown that the breakdown voltage of the weakest point in a winding can be forecast nondestructively by insulation resistance tests at increasing voltages. A new method of stating dielectric-absorption effect is suggested and its value as a criterion of mechanical strength of hidden insulation is indicated. The detection and location of internal voids in coils, by ionization tests of established types, are described and comment is made on the relation of such voids to interturn and interstrand faults. The simple test equipment used is briefly described. A routine test program based on the findings is outlined. Proposals are given for lines of further study. Tests to date have been concerned chiefly with the varnish-bondedmica type of insulation, but there are indications of wider applicability.