The first of the modifications referred to above for the D.C. machine, involves the introduction of an iron or magnetic material into the armature or moving-coil part of the assembly. The coil made up from insulated wire or strip, is wound onto an iron armature which is carried on the shaft. The magnet system is also provided with iron pole-shoes or extensions as shown in air is now reduced to two small air-gaps, the remainder being through the iron of the armature and field system; the flux density of B value in the air-gaps is increased and the conductors will therefore cut a stronger field. Again as the air-gaps are now small and of constant width, the flux lines will cross them as shown and the field will be uniform over the pole-faces. The moving conductors thus pass from a small arc with substantially no magnetic flux into a large arc of constant flux density. The flux lines are seen to be radial in the gaps and are cut at right angles for most of the distance under the pole. The E.M.F. waveform is now as shown in the diagram (Fig 92.) ie it is proportional to the flux density through which it passes