PROPERTIES 205
of soft carbon heat treated from 1800"-3000Β°C after introduction of acceptors such as boron and bisulfate residual compound. Although the two types of acceptors differ radically in their effect on the absolute value of the resistivity, it has been found that introduction of acceptors to a great extent has a similar effect on galvanomagnetic properties as a corresponding decrease in heat treatment temperature. The total temperature change in relative resistivity decreases with introduction of acceptors, the resistivity curves retaining however some curvature of the type proper for the higher heat treatment and absent for the nondoped original material of correspondingly lower HTT. There are some differences to be noticed also in the Hall coefficient curves. In general, however, one can say that the main factor controlling the type of temperature dependence observed seems to be the position of the Fermi level, the carrier scattering (mobility) either being little temperature dependent, or having about the same temperature dependence for a wide range of crystallite sizes. l Supported by the Office of Naval Rcseaxb. t On leave of absence from Showa Denko Co.. Japan. 25. Field dependence of magnetoresistivity and Hall coefficient for carbons at various temperatures.*