▁the ▁l ens ▁y oke . ▁The ▁co ils ▁can ▁contain ▁a ▁variable ▁current , ▁but ▁typically ▁use ▁high ▁volt ages , ▁and ▁therefore ▁require ▁significant ▁ins ulation ▁in ▁order ▁to ▁prevent ▁short - circ uit ing ▁the ▁l ens ▁components . ▁Th erm al ▁distrib utors ▁are ▁placed ▁to ▁ensure ▁the ▁extra ction ▁of ▁the ▁heat ▁generated ▁by ▁the ▁energy ▁lost ▁to ▁resistance ▁of ▁the ▁co il ▁wind ings . ▁The ▁wind ings ▁may ▁be ▁water - co o led , ▁using ▁a ▁ch illed ▁water ▁supply ▁in ▁order ▁to ▁facil itate ▁the ▁removal ▁of ▁the ▁high ▁thermal ▁duty . ▁ ▁A pert ures ▁A pert ures ▁are ▁ann ular ▁met all ic ▁pl ates , ▁through ▁which ▁electrons ▁that ▁are ▁further ▁than ▁a ▁fixed ▁distance ▁from ▁the ▁opt ic ▁axis ▁may ▁be ▁ex cluded . ▁These ▁consist ▁of ▁a ▁small ▁met all ic ▁disc ▁that ▁is ▁sufficiently ▁thick ▁to ▁prevent ▁electrons ▁from ▁passing ▁through ▁the ▁disc , ▁whilst ▁perm itting ▁ax ial ▁electrons . ▁This ▁permission ▁of ▁central ▁electrons ▁in ▁a ▁T EM ▁causes ▁two ▁effects ▁simultaneously : ▁first ly , ▁a pert ures ▁decrease ▁the ▁beam ▁intensity ▁as ▁electrons ▁are ▁filtered ▁from ▁the ▁beam , ▁which ▁may ▁be ▁desired ▁in ▁the ▁case ▁of ▁beam ▁sensitive ▁samples . ▁Second ly , ▁this ▁filtering ▁removes ▁electrons ▁that ▁are ▁scattered ▁to ▁high ▁angles , ▁which ▁may ▁be ▁due ▁to ▁unw anted ▁processes ▁such ▁as ▁spher ical ▁or ▁chrom atic ▁aber r ation , ▁or ▁due ▁to ▁diff raction ▁from ▁interaction ▁within ▁the ▁sample