Conductivity  of  Metallic  Oxides.  509 
In  some  cases  the  resistance  of  the  oxide  was  obtained  by 
measuring  the  current  passing  through  it  under  a  known 
Fig*.  1. — Electric  Furnace. 
A  A  are  the  leads  to  the  heating-coil ;  B  B  are  the  platinum  wires 
from  the  electrodes  ;  C  C  the  thermocouple  wires. 
E.M.F.  by  means  of    a  delicate   d'Arsonval  galvanometer  : 
but  more  generally  the  resistance  was  measured  directly  by  a 
