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Iron cores are used in radio-frequency coils primarily to obtain increased inductance. The amount of inductance increase due to the iron core depends upon the physical dimensions of the coil and the core, and on the effective permeability of the core. In addition to the inductance increase, the iron core increases the effective radio-frequency resistance of the coil due to hysteresis and eddy-current losses in the iron. This paper describes a method of measuring the effective permeability and loss factor of iron at radio frequencies, and develops expressions by means of which the characteristics of a given coil can be predicted when the iron constants are known. The measurement method described is employed to measure several cores. Sample calculations and curves are given to illustrate the use of the formulas developed herein.