When tesla suggested the ac motor, his teacher humiliated him and said he was trying to create what?

While studying at the Polytechnic school Tesla's physics teacher showed the class a new Gramme dynamo that could be used as both a motor and a generator. After watching it for a time, Tesla suggested it is possible to get rid of sparking connections known as commutators. His professor jokingly said that he would be like building a perpetual motion machine!