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..Having found that genuinely macroscopic forces are available from the magnetic-field interactions of current-carrying conductors, the quest for practical motor action would, nevertheless, probably be doomed to failure. Although you could dream up ingenious ways to reverse currents, and therefore the direction of magnetic fields, motors based on the interaction between two single conductors would have features best described as toylike. The geometry of such an arrangement would be embarrassingly awkward, and such a motor would probably serve little more than to pull its own weight. What is needed is a means by which the magnetic lines of force can be collected and focused within the confines of a small area or volume. With such a technique, you would have at your disposal a relatively strong magnetic field in a compacted space. This is because the field strength is determined by the density of the magnetic lines of force. Thus, in a bar magnet, the so-called poles are those regions external to the bar itself, where the density of the magnetic field is the greatest.
You might experiment with the idea of paralleling conductors, and indeed, this would be a step forward. Consider the solenoid, an arrangement in which the conductor consists of a layer of coiled wire, or many such layers. In Fig. 13A, the collecting and focusing action of the solenoid is shown. In its ultimate effect, the current-carrying solenoid closely duplicates the behavior of the bar magnet as far as the external magnetic field is concerned. This, indeed, led to early notions of magnetism as a phenomenon stemming from the overall effects of a tremendous number of tiny current loops. At first, this was attributed to the orbital motions of certain electrons, but later investigations suggest electron spin as the basis for magnetism in ferromagnetic materials.
FIG. 13 Increasing the magnetic field strength of a solenoid.
A. The air-core solenoid. B. Same solenoid with core of iron or other ferromagnetic material.
The solenoid provides a controllable source of magnetism. What more can you desire in your search for techniques suitable for producing motor action? It turns out that you need even more powerful concentrators of magnetic lines of force than the air-core solenoid. This is true even though you might be willing to deploy thousands of turns and to utilize current magnitudes up to the maximum capacity of the wire. Essentially, you want to retain the basic action of the solenoid, but to obtain more magnetic lines of force per ampere. Fortunately, the greed for more output and less input can be easily satisfied.