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It is shown that the frequency of a resonant electromagnetic cavity is perturbed by inserting a metallic sphere, needle, or disk of dimensions small compared to a wavelength by an amount depending upon the local electric and magnetic field at the position of the perturbing object. This perturbation is calculated for ellipsoidal objects of needle‐shaped, spherical, and disk‐shaped form. The perturbations by the different objects depend upon different components of electric and magnetic fields, and by combining measurements with all three, it is in theory possible to measure all the field components. Experimental checks of the calculations are described, resulting in satisfactory agreement between theory and experiment except with the needles, in which the perturbation is very sensitive to the precise shape of the object, and the needles used were not accurate enough ellipsoids to give satisfactorily quantitative results.