▁hot ▁metal ▁st rap ▁( or ▁wire ). ▁They ▁are ▁then ▁acceler ated ▁and ▁formed ▁into ▁a ▁narrow ▁conver gent ▁beam ▁by ▁an ▁electric ▁field ▁produced ▁by ▁three ▁electro des : ▁the ▁electron ▁em itting ▁st rap , ▁the ▁c ath ode ▁connected ▁to ▁the ▁negative ▁pole ▁of ▁the ▁high ▁( ac cel er ating ) ▁voltage ▁power ▁supply ▁( 3 0 ▁- ▁ 2 0 0   k V ) ▁and ▁the ▁positive ▁high ▁voltage ▁electro de , ▁the ▁an ode . ▁There ▁is ▁a ▁third ▁electro de ▁charged ▁neg atively ▁with ▁respect ▁to ▁the ▁c ath ode , ▁called ▁the ▁We hn elt ▁or ▁control ▁electro de . ▁Its ▁negative ▁potential ▁controls ▁the ▁portion ▁of ▁em itted ▁electrons ▁entering ▁into ▁the ▁acceler ating ▁field , ▁i . e ., ▁the ▁electron - be am ▁current . ▁ ▁After ▁passing ▁the ▁an ode ▁opening , ▁the ▁electrons ▁move ▁with ▁constant ▁speed ▁in ▁a ▁slightly ▁diver gent ▁cone . ▁For ▁techn ological ▁applications ▁the ▁diver gent ▁beam ▁has ▁to ▁be ▁focused , ▁which ▁is ▁realized ▁by ▁the ▁magnetic ▁field ▁of ▁a ▁co il , ▁the ▁magnetic ▁foc using ▁l ens . ▁ ▁For ▁proper ▁function ing ▁of ▁the ▁electron ▁gun , ▁it ▁is ▁necessary ▁that ▁the ▁beam ▁be ▁perfectly ▁adjust ed ▁with ▁respect ▁to ▁the ▁optical ▁axes ▁of ▁the ▁acceler ating ▁elect rical ▁l ens ▁and ▁the ▁magnetic ▁foc using ▁l ens . ▁This ▁can ▁be ▁done ▁by ▁applying ▁a ▁magnetic ▁field ▁of ▁some ▁specific ▁radial ▁direction ▁and ▁strength ▁per pend icular ▁to