We have constructed a3Heimmersion cell for dielectric measurements of insulating amorphous solids in a nuclear demagnetization cryostat at temperatures between 500 &mgr;K and 150 mK. The samples are directly immersed in3Hewith two heat exchangers per electrode which are thermally isolated from each other and have a very low capacitance to ground. The cell incorporates a195Ptpulsed NMR thermometer with a novel superconducting magnet and a3Heviscometer for calibration. ©1997 American Institute of Physics.