The Metal Varistor is the core of the arrester, whose main components is zinc oxide and with a small quantity of bismuth oxide, fluoride oxide, manganous carbonate, antimony oxide and other gleam of additive and made with high temperature. 

Metal Oxide Varistor's microstructure mainly constitute of zinc oxide grain, grain boundary layer and chienshih phase, and its electric conduction principle lies that zinc oxide grain has good electro conductiility, and the voltage to the zinc oxide is almost act on high resistant grain boundary layer. Varistor's nonlinearity mainly comes from grain boundary layer ( the main ingredient is bismuth oxide) Chienshih phase scattered throughout the grain boundary layer which is the multiple oxide of zinc oxide and bismuth oxide. Its function lies in restraining the growing of zinc oxide grain to make the better nonlinearity.