ode ▁- ▁the ▁source ▁of ▁free ▁electrons ▁ ▁Con du ction ▁electrons ▁( th ose ▁not ▁bound ▁to ▁the ▁nucle us ▁of ▁atoms ) ▁move ▁in ▁a ▁cry stal ▁lattice ▁of ▁met als ▁with ▁veloc ities ▁distributed ▁according ▁to ▁G auss ' s ▁law ▁and ▁depending ▁on ▁temperature . ▁They ▁cannot ▁leave ▁the ▁metal ▁unless ▁their ▁kin etic ▁energy ▁( in ▁e V ) ▁is ▁higher ▁than ▁the ▁potential ▁bar rier ▁at ▁the ▁metal ▁surface . ▁The ▁number ▁of ▁electrons ▁ful fill ing ▁this ▁condition ▁increases ▁expon entially ▁with ▁increasing ▁temperature ▁of ▁the ▁metal , ▁following ▁Richard son ' s ▁rule . ▁ ▁As ▁a ▁source ▁of ▁electrons ▁for ▁electron - be am ▁w eld ers , ▁the ▁material ▁must ▁ful fill ▁certain ▁requirements : ▁to ▁achieve ▁high ▁power ▁density ▁in ▁the ▁beam , ▁the ▁emission ▁current ▁density ▁[ A / mm 2 ], ▁hence ▁the ▁working ▁temperature , ▁should ▁be ▁as ▁high ▁as ▁possible , ▁to ▁keep ▁ev ap oration ▁in ▁vac u um ▁low , ▁the ▁material ▁must ▁have ▁a ▁low ▁enough ▁v ap our ▁pressure ▁at ▁the ▁working ▁temperature . ▁The ▁em itter ▁must ▁be ▁mechan ically ▁stable , ▁not ▁chem ically ▁sensitive ▁to ▁g ases ▁present ▁in ▁the ▁vac u um ▁atmosphere ▁( like ▁o xygen ▁and ▁water ▁v ap our ), ▁easily ▁available , ▁etc . ▁ ▁These ▁and ▁other ▁conditions ▁limit ▁the ▁choice ▁of ▁material ▁for ▁the ▁em itter ▁to ▁met als ▁with ▁high ▁mel ting ▁points , ▁pract ically ▁to ▁only ▁two : ▁tant al um