  We report a technique for applying a dc voltage or current bias to the center
conductor of a high-quality factor superconducting microwave cavity without
significantly disturbing selected cavity modes. This is accomplished by
incorporating dc bias lines into the cavity at specific locations. The measured
S-matrix parameters of the system are in good agreement with theoretical
predictions and simulations. We find that at 4 K the quality factor of the
cavity degrades by less than 1% under the application of a dc bias.
