We report measurements of the energy efficiency of currents induced from a primary circuit hosting brief high-voltage pulses into near oscillating secondary circuits. The size of all circuits is of the order of 10–100 cm, and the distance between primary and secondary also lies in this range. The primary circuit comprises two aluminum electrodes holding a disk made either of metal, or special silicon-graphite, or YBCO. This element of the primary, also called “emitter,” is kept in a cryostat at temperatures between 77 and 300 K. When primary voltage and temperature are varied (keeping the primary impedance constant), the energy transfer efficiency is found to be constant for metal emitters, but it exhibits clear variations for the graphite/SiC and YBCO emitters. Such variations are not... explained by usual models of conductivity and superconductivity, complemented by Maxwell equations. We discuss possible interpretations of the phenomenon.