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Hardy and Whitehead (I), apparently working from an NMR perspective, have described a novel sample resonator for the range of 200 to 2000 MHz. This type of resonator was described as early as 1940 (2) in the context of magnetron designs and more recently in the literature of heavy-ion particle accelerators (3). We describe here the design and use of structures of this type for ESR spectroscopy between 1 and 10 GHz. As will be apparent, it offers some remarkable advantages. We have previously published data at 3.8 GHz (4). Figure I shows one of our designs and also defines the notation. Various names have been used for this structure: we introduce here the name “loop-gap.” The loop is the inductive element, a, surrounding the sample, while the gaps, b, are capacitive elements. We find it convenient to employ nouns describing the two key elements of the resonator as a compound name for the structure itself. Previous workers have used single-gap resonators. We have discovered that the number of gaps can be increased and that the equivalent circuit of the resonator has the capacities of the individual gaps connected in series. This is a lumped circuit resonator. The dimensions of the structure should be small compared with onequarter wavelength, X/4. Capacitive and inductive elements are separated in space, and to a fair approximation the rf magnetic field exists inside the loop and the rf electric field inside the gaps. The classical expressions for the inductance and capacitance, assuming n identical gaps, are twz c=-h7rr 2 tn’L=----Z and the resonant frequency is given by