Hele-Shaw & Hay. 
Phil. Trans., A., Vol. 195 , Plate 20 . 
33 
34 
Hollow Square Shield. Permeability = 100. 
Induction in Air-gap and Teeth of Toothed- 
core Armature. Permeability = 100. 
Field between Tapered Pole-pieces of Attraction of Armature by Pole-piece. 
Electromagnet. 
