Thermal Conductivity and Heat Capacity of 7740 Pyrex below 4°K and in Magnetic Fields to 90 kG

The thermal conductivity of Pyrex 7740 glass has been measured over the temperature range 0.4 to 4°K and in magnetic fields from 0 to 90 kG. As expected, the conductivity was unaffected by the field. The glass had an iron content of 0.02%. The heat capacity of Pyrex, at constant magnetic fields, has been measured over the above range of temperature and field. There was a considerable field effect due to the iron content. The variation of temperature with field was measured on a series of isentropes and this enabled the evaluation of the entropy. The change of temperature of Pyrex during adiabatic demagnetization at various values of the entropy is shown as a graph for this typical sample. A description of the apparatus used for the measurement is included.