We have constructed a He-3 immersion cell for dielectric measurements of insulating amorphous solids in a nuclear demagnetization cryostat at temperatures between 500 mu K and 150 mK. The samples are directly immersed in He-3 with two heat exchangers per electrode which are thermally isolated from each other and have a very low capacitance to ground. The cell incorporates a Pt-195 pulsed NMR thermometer with a novel superconducting magnet and a He-3 viscometer for calibration. (C) 1997 American Institute of Physics.