Abstract: An ESR spectrometer is equipped with a split-ring resonator comprising an electrically conductive cylinder or loop which may be excited by a microwave antenna. The conductive cylinder or loop is secured to a surface of an insulating sheet whereby the other surface may be brought to bear upon the specimen being examined. Preferredly, the insulating sheet comprises two insulating sheets with a Faraday shield sandwiched therebetween whereby the magnetic field generated by the microwave energy applied to the loop leaks to the surface of the specimen but the electric field is inhibited.

Abstract: The prior art split-ring resonator for use in an electron spin resonance (ESR) spectrometer comprises a circular cylinder made from an electrically conductive material and housed in a metallic tube. The cylinder is provided with a split extending axially of the cylinder. A sample to be investigated is inserted into the cylinder in the axial direction. In accordance with the invention, a shielding member which is wider than said slit and made from an electrically conductive material is disposed so as to shield the slit from the inside of the cylinder, whereby the resonance frequency of the resonator is not significantly affected by the presence or absence of the sample or by the amount of the inserted sample.