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A criterion for describing the degree of balance of transformers is proposed and techniques for measuring balance in accordance with this criterion are described. Such a criterion provides a means for assigning a numerical value to the degree to which a transformer, under a particular set of operating conditions, can convert an unbalanced signal into a balanced signal or suppress unwanted longitudinal signals. It can be applied to any balanced passive system or any grounding arrangement that results in a balanced output. Although it is difficult to make a direct measurement of balance in accordance with this criterion, an indirect technique is proposed which is theoretically rigorous and permits measurements to be made under conditions that closely approximate the anticipated use conditions. These indirect measurements are relatively simple and satisfactory accuracy can be achieved with commercially available test equipment. The theory is verified by experimental data that show excellent agreement between the direct method and the proposed simplified method.