| Popis: |
The principal electrical conductivities (${\ensuremath{\sigma}}_{a}$ in the basal plane and ${\ensuremath{\sigma}}_{c}$ perpendicular to the basal plane) of graphite crystals isolated from several North American marbles were studied. By means of measurements of potential distributions on the surfaces of crystals carrying electric current, it was conclusively shown that $\frac{{\ensuremath{\sigma}}_{a}}{{\ensuremath{\sigma}}_{c}}$ for these crystals is about several hundred at room temperature. By direct measurement ${\ensuremath{\sigma}}_{a}$ was found to be (2.6\ifmmode\pm\else\textpm\fi{}0.2)\ifmmode\times\else\texttimes\fi{}${10}^{4}$ (ohm ${\mathrm{c}\mathrm{m})}^{\ensuremath{-}1}$ and ${\ensuremath{\sigma}}_{c}$ to vary from 150 to 230 (ohm ${\mathrm{c}\mathrm{m})}^{\ensuremath{-}1}$ (\ifmmode\pm\else\textpm\fi{}20 percent) at room temperature. Below room temperature it was found for two crystals that ${\ensuremath{\sigma}}_{a}$ increased slowly to liquid nitrogen temperatures, then followed a ${T}^{\ensuremath{-}1}$ law to about 15\ifmmode^\circ\else\textdegree\fi{}K, and again slowly increased in the region 15\ifmmode^\circ\else\textdegree\fi{}K to 4.2\ifmmode^\circ\else\textdegree\fi{}K. Two other crystals for which ${\ensuremath{\sigma}}_{c}$ was measured at low temperatures did not behave alike. For both, ${\ensuremath{\sigma}}_{c}$ decreased as the temperature was lowered, reached a minimum above liquid nitrogen temperatures, and then started increasing, the one showing a ${T}^{\ensuremath{-}1}$ behavior, the other increasing more slowly. For both, the rate of increase declined markedly in the region 15\ifmmode^\circ\else\textdegree\fi{}K to 4.2\ifmmode^\circ\else\textdegree\fi{}K. |
