▁product ▁of ▁the ▁radial ▁component ▁of ▁induction ▁Br ▁and ▁ax ial ▁component ▁of ▁velocity ▁va ▁is ▁a ▁force ▁per pend icular ▁to ▁those ▁vectors , ▁causing ▁the ▁electron ▁to ▁move ▁around ▁the ▁axis . ▁Add itional ▁effect ▁of ▁this ▁motion ▁in ▁the ▁same ▁magnetic ▁field ▁is ▁another ▁force ▁F ▁orient ed ▁radial ly ▁to ▁the ▁axis , ▁which ▁is ▁responsible ▁for ▁the ▁foc using ▁effect ▁of ▁the ▁magnetic ▁l ens . ▁The ▁resulting ▁traject ory ▁of ▁electrons ▁in ▁the ▁magnetic ▁l ens ▁is ▁a ▁curve ▁similar ▁to ▁a ▁hel ix . ▁In ▁this ▁context ▁it ▁should ▁be ▁mentioned ▁that ▁variations ▁of ▁foc al ▁length ▁( ex c iting ▁current ) ▁cause ▁a ▁slight ▁rotation ▁of ▁the ▁beam ▁cross - section . ▁ ▁Be am ▁def lection ▁system ▁ ▁As ▁mentioned ▁above , ▁the ▁beam ▁spot ▁should ▁be ▁very ▁precisely ▁position ed ▁with ▁respect ▁to ▁the ▁joint ▁to ▁be ▁w eld ed . ▁This ▁is ▁commonly ▁accomplished ▁mechan ically ▁by ▁moving ▁the ▁work pie ce ▁with ▁respect ▁to ▁the ▁electron ▁gun , ▁but ▁sometimes ▁it ▁is ▁prefer able ▁to ▁def lect ▁the ▁beam ▁instead . ▁Most ▁often ▁a ▁system ▁of ▁four ▁co ils ▁position ed ▁symmet r ically ▁around ▁the ▁gun ▁axis ▁behind ▁the ▁foc using ▁l ens , ▁producing ▁a ▁magnetic ▁field ▁per pend icular ▁to ▁the ▁gun ▁axis , ▁is ▁used ▁for ▁this ▁purpose . ▁ ▁There ▁are ▁more ▁practical ▁reasons ▁why ▁the ▁most ▁appropriate ▁def lection ▁system ▁is ▁used ▁in ▁TV ▁C RT ▁or ▁PC ▁mon itors . ▁This ▁applies ▁to