▁are ▁ 2 6 2 = 2 7 6 ▁pure ▁periods ▁of ▁the ▁length ▁ 1 2 ▁x ▁ 2 3 ▁x ▁ 2 5 ▁x ▁ 2 6 ▁= ▁ 3 1 3 , ▁ 9 5 0 ▁as ▁long ▁as ▁the ▁number ▁of ▁active ▁p ins ▁on ▁each ▁of ▁the ▁pin ▁whe els ▁is ▁prime ▁to ▁ 2 ▁and ▁ 1 3 . ▁This ▁last ▁condition ▁should ▁be ▁laid ▁down ▁in ▁the ▁ci pher ▁reg ulations ; ▁otherwise ▁the ▁ 6 7 6 ▁periods ▁would ▁be ▁further ▁broken ▁in ▁a ▁manner ▁easily ▁seen . ▁ ▁Things ▁are ▁much ▁more ▁complicated ▁in ▁the ▁case ▁of ▁Model ▁b ). ▁Investig ations ▁into ▁this ▁problem ▁in ▁the ▁winter ▁of ▁ 1 9 4 2 / 1 9 4 3 ▁were ▁only ▁partly ▁successful ; ▁above ▁all ▁it ▁was ▁not ▁possible ▁exactly ▁to ▁calculate ▁the ▁lengths ▁of ▁the ▁pure ▁periods ▁and ▁the ▁pre - period s . ▁Est im ates ▁which ▁were ▁quite ▁ade qu ate ▁for ▁practical ▁purposes ▁were ▁however ▁given . ▁B ugg isch ▁could ▁not ▁remember ▁the ▁details ▁of ▁these ▁somewhat ▁investig ations . ▁The ▁extraordinary ▁length ▁of ▁many ▁pre - period s ▁( length s ▁of ▁some ▁thousands ▁were ▁not ▁un common ) ▁and ▁the ▁comp lication ▁of ▁their ▁branches ▁were ▁remarkable . ▁The ▁general ▁type ▁can ▁be ▁illustrated ▁by ▁the ▁following ▁diagram : ▁ ▁In ▁this ▁circle ▁represents ▁the ▁pure ▁period ▁and ▁the ▁straight ▁lines ▁the ▁pre - period s . ▁There ▁were ▁usually ▁several ▁pure ▁periods , ▁each ▁one