▁bore ▁and ▁into ▁the ▁spec imen . ▁Such ▁designs ▁are ▁typically ▁unable ▁to ▁be ▁t ilt ed ▁without ▁blocking ▁the ▁beam ▁path ▁or ▁inter fer ing ▁with ▁the ▁objective ▁l ens . ▁ ▁Electron ▁gun ▁ ▁The ▁electron ▁gun ▁is ▁formed ▁from ▁several ▁components : ▁the ▁fil ament , ▁a ▁bias ing ▁circuit , ▁a ▁We hn elt ▁cap , ▁and ▁an ▁extra ction ▁an ode . ▁By ▁connecting ▁the ▁fil ament ▁to ▁the ▁negative ▁component ▁power ▁supply , ▁electrons ▁can ▁be ▁" p ump ed " ▁from ▁the ▁electron ▁gun ▁to ▁the ▁an ode ▁plate ▁and ▁the ▁T EM ▁column , ▁thus ▁comple ting ▁the ▁circuit . ▁The ▁gun ▁is ▁designed ▁to ▁create ▁a ▁beam ▁of ▁electrons ▁exit ing ▁from ▁the ▁assembly ▁at ▁some ▁given ▁angle , ▁known ▁as ▁the ▁gun ▁diver gence ▁semi - angle , ▁α . ▁By ▁construct ing ▁the ▁We hn elt ▁cyl inder ▁such ▁that ▁it ▁has ▁a ▁higher ▁negative ▁charge ▁than ▁the ▁fil ament ▁itself , ▁electrons ▁that ▁exit ▁the ▁fil ament ▁in ▁a ▁diver ging ▁manner ▁are , ▁under ▁proper ▁operation , ▁forced ▁into ▁a ▁conver ging ▁pattern ▁the ▁minimum ▁size ▁of ▁which ▁is ▁the ▁gun ▁c ros so ver ▁diameter . ▁ ▁The ▁therm ionic ▁emission ▁current ▁density , ▁J , ▁can ▁be ▁related ▁to ▁the ▁work ▁function ▁of ▁the ▁em itting ▁material ▁via ▁Richard son ' s ▁law ▁ ▁where ▁A ▁is ▁the ▁Richard son ' s ▁constant , ▁ Φ ▁is ▁the ▁work ▁function ▁and ▁T ▁is ▁the ▁temperature ▁of ▁the ▁material