. ▁ ▁This ▁equation ▁shows ▁that ▁in ▁order ▁to ▁achieve ▁sufficient ▁current ▁density ▁it ▁is ▁necessary ▁to ▁heat ▁the ▁em itter , ▁taking ▁care ▁not ▁to ▁cause ▁damage ▁by ▁application ▁of ▁excess ive ▁heat . ▁For ▁this ▁reason ▁materials ▁with ▁either ▁a ▁high ▁mel ting ▁point , ▁such ▁as ▁t ung sten , ▁or ▁those ▁with ▁a ▁low ▁work ▁function ▁( La B 6 ) ▁are ▁required ▁for ▁the ▁gun ▁fil ament . ▁Furthermore , ▁both ▁l anth an um ▁hex abor ide ▁and ▁t ung sten ▁therm ionic ▁sources ▁must ▁be ▁he ated ▁in ▁order ▁to ▁achieve ▁therm ionic ▁emission , ▁this ▁can ▁be ▁achieved ▁by ▁the ▁use ▁of ▁a ▁small ▁resist ive ▁strip . ▁To ▁prevent ▁thermal ▁shock , ▁there ▁is ▁often ▁a ▁delay ▁enfor ced ▁in ▁the ▁application ▁of ▁current ▁to ▁the ▁tip , ▁to ▁prevent ▁thermal ▁grad ients ▁from ▁dam aging ▁the ▁fil ament , ▁the ▁delay ▁is ▁usually ▁a ▁few ▁seconds ▁for ▁La B 6 , ▁and ▁significantly ▁lower ▁for ▁t ung sten . ▁ ▁Electron ▁l ens ▁ ▁Electron ▁l enses ▁are ▁designed ▁to ▁act ▁in ▁a ▁manner ▁em ulating ▁that ▁of ▁an ▁optical ▁l ens , ▁by ▁foc using ▁parallel ▁electrons ▁at ▁some ▁constant ▁foc al ▁distance . ▁Electron ▁l enses ▁may ▁operate ▁electro stat ically ▁or ▁magnet ically . ▁The ▁majority ▁of ▁electron ▁l enses ▁for ▁T EM ▁use ▁elect romagnet ic ▁co ils ▁to ▁generate ▁a ▁convex ▁l ens . ▁The ▁field ▁produced ▁for ▁the ▁l ens ▁must ▁be ▁radial ly ▁symmet rical ,