Computing and accessing normals from cells instead of points

Hi all, I am very new to using Slicer. I would like to generate normals for each cell instead of the default point normals.

I have been able to set the cell normals flag on (and the point cell normals flag off) in the python interactor using vtk.vtkPolyDataNormals().ComputeCellNormalsOn() however I am not sure how to actually generate an array of normals after the flags have been set. Using getNode("Output Volume").GetPolyData().GetCellData().GetArray("Normals") yields nothing.

Any suggestions on how to generate and access cell normals?