The ventilating duct spacer must extend from the top of each tooth to the inside of the armature lamination so that neither the teeth nor the inside edge of the armature lamination will flare and close the duct when the punchings are pressed together. Loose armature laminations will vibrate and porduce a buzzing noise, because of the flux reversals in the armature core. The construction of the ventilating duct spacer is similar to that of the tooth support shown in Fig. 4.