The electrical properties of graphite

The Hall coefficient and resistivity of a range of polycrystalline graphites with different crystal sizes and a single crystal of Travancore graphite have been measured over a wide range of temperature. The number of free electrons has been found to be approximately 6 $\times $ 10$^{18}$ per cm$^{3}$ at room temperature; the variation with temperature cannot be accurately determined. The deficit of electrons in poorly crystalline graphite gives rise to positive Hall coefficients. Quenching removes electrons, and a study of this process has enabled the ratio of the mobilities of positive holes and electrons to be estimated at 0$\cdot $80. An interesting effect has been observed in the variation of the Hall coefficient of the single crystal with field; no satisfactory explanation has been found for this phenomenon. The resistivity of polycrystalline graphite depends on the density and on the orientation and size of the crystals. From the variation of resistivity with temperature and the size of the crystals, the mean free path due to thermal scattering has been found to be 2350 angstrom at 273 degrees K; the variation of mean free path with temperature has been deduced. The product of effective mass and velocity of the free electrons has been determined as a function of temperature; the accuracy is limited by uncertainties in the number of free electrons.