▁agree ▁with ▁the ▁corresponding ▁ones ▁on ▁the ▁En igma ▁inter change able ▁working ▁between ▁both ▁machines ▁is ▁possible . ▁A ▁sens ing - point ▁is ▁also ▁provided ▁opposite ▁wheel ▁III . ▁If ▁the ▁turn - over ▁not ch ▁on ▁the ▁ring ▁of ▁III ▁is ▁touched ▁by ▁it , ▁then ▁I ▁turns ▁on ▁one ▁place ▁when ▁the ▁next ▁letter ▁is ▁key ed . ▁If ▁this ▁movement ▁coinc ides ▁with ▁the ▁step ▁caused ▁by ▁N 1 , ▁this ▁again ▁results ▁in ▁the ▁single ▁step . ▁Thus ▁the ▁possibility ▁of ▁inter change able ▁working ▁with ▁the ▁En igma ▁remains . ▁In ▁addition , ▁the ▁machine ▁now ▁gets ▁a ▁fourth ▁wheel , ▁which ▁is ▁placed ▁between ▁III ▁and ▁U ▁and ▁does ▁not ▁move ▁on ▁when ▁a ▁key ▁is ▁touched . ▁It ▁corresponds ▁to ▁the ▁fourth ▁wheel ▁on ▁the ▁Naval ▁En igma ▁and ▁is ▁used ▁for ▁inter change able ▁working ▁with ▁this ▁machine . ▁In ▁the ▁summer ▁of ▁ 1 9 4 4 , ▁Karl ▁Stein ▁of ▁OK W / Ch i ▁told ▁B ugg isch ▁that ▁the ▁reci pro cal ▁influen cing ▁of ▁the ▁whe els ▁was ▁to ▁be ▁alter ed ▁in ▁some ▁way . ▁B ugg isch ▁could ▁not ▁remember ▁the ▁details ▁but ▁nothing ▁fundamental ▁on ▁the ▁principle ▁of ▁the ▁machine ▁described ▁under ▁c ) ▁was ▁changed . ▁Inter change able ▁working ▁with ▁army ▁and ▁naval ▁En igma ▁remained ▁possible . ▁ ▁Investig ations ▁in ▁Period icity ▁In ▁the ▁case ▁of ▁model ▁first ▁a ) ▁above , ▁the ▁question ▁of ▁period icity ▁is ▁elementary , ▁there