▁circle , ▁forming ▁a ▁tor us ▁( a ▁ring ▁or ▁don ut ). ▁In ▁this ▁case , ▁the ▁particles ▁will ▁still ▁be ▁conf ined ▁to ▁the ▁middle ▁of ▁the ▁cyl inder , ▁and ▁even ▁if ▁they ▁move ▁along ▁it ▁they ▁would ▁never ▁exit ▁the ▁ends ▁- ▁they ▁would ▁circle ▁the ▁appar atus ▁end lessly . ▁However , ▁F erm i ▁noted ▁a ▁problem ▁with ▁this ▁arrangement ; ▁consider ▁a ▁series ▁of ▁circular ▁magnet s ▁with ▁the ▁tor oid al ▁conf in ement ▁area ▁thread ed ▁through ▁their ▁cent ers , ▁the ▁magnet s ▁will ▁be ▁closer ▁together ▁on ▁the ▁inside ▁of ▁the ▁ring , ▁with ▁a ▁stronger ▁field . ▁Part icles ▁in ▁such ▁a ▁system ▁will ▁dr ift ▁up ▁or ▁down ▁across ▁the ▁tor us . ▁ ▁The ▁solution ▁to ▁this ▁problem ▁is ▁to ▁add ▁a ▁secondary ▁magnetic ▁field ▁at ▁right ▁angles ▁to ▁the ▁first . ▁The ▁two ▁magnetic ▁fields ▁will ▁mix ▁to ▁produce ▁a ▁new ▁combined ▁field ▁that ▁is ▁hel ical , ▁like ▁the ▁stri pes ▁on ▁a ▁bar ber ▁pole . ▁A ▁particle ▁orbit ing ▁such ▁a ▁field ▁line ▁will ▁find ▁itself ▁near ▁the ▁outside ▁of ▁the ▁conf in ement ▁area ▁at ▁some ▁times , ▁and ▁near ▁the ▁inside ▁at ▁others . ▁Although ▁a ▁test ▁particle ▁would ▁always ▁be ▁dr ifting ▁up ▁( or ▁down ) ▁compared ▁to ▁the ▁field , ▁since ▁the ▁field ▁is ▁rot ating , ▁that ▁dr ift ▁will , ▁compared ▁to ▁the ▁conf in ement ▁chamber , ▁be ▁up ▁or ▁down , ▁in ▁or ▁out , ▁depending