The earlier types of Edison dynamo became obsolete as improved designs came along. Then Dr Hopkinson improved performance. Instead of the multiple field-magnet limbs, each wound with magnetising coils which join the pole-pieces to the yoke in the older machines, Dr. Hopkinson used only one limb on each side, solidly connected to the pole-piece at one end and the yoke at the other. The cross-section of the iron cores of these limbs is greater than the cross-section of the iron in the older multiple limbs, and the cores are also shorter in length. In addition, the iron yoke across the top is made much heavier. The result of these changes is that the same dead weight of iron is more advantageously arranged for being readily magnetised because the magnetic circuit is both shortened in length and has its cross-sectional area increased throughout. The magnet cores and pole-pieces in this machine consist of a single forging, and in later machines the magnets were wound with wire of square section, more of which can be packed into a given space than is possible with ordinary round wire. Besides altering the field-magnets, Dr. Hopkinson modified the armature of the machine, getting more iron into it, thus diminishing the magnetic reluctance in this part of the magnetic circuit.