This section is from the book "Amateur Work Magazine Vol6". Also available from Amazon: Amateur Work.
Ira M. Cushing
The magnetic circuit has been found to have many properties in common with the electric circuit. There is a difference of magnetic potential causing lines of force to flow from the north to the south pole, just as there is a difference of electric potential between the terminals of a battery. The total power of the magnet or magneto motive-force (M. M. F.) is equal to the sum of the M. M. F. of each turn of the coil, just as the total electromotive force of a battery is the sum of the E. M. F. of each cell when connected in series.
Some materials conduct magnetism better than others, in the same manner that some conduct the electric current better than others. Also, the reluctance, which is the resistance in materials to the passage of magnetism, is proportional to the length of the path, and inversly proportional to the cross section of the path, which is true of the electric circuit.
As stated above, materials have a reluctance to the passage of magnetism, and this is called Reluctivity. The opposite to this is called Permeability. Various definitions have been given for this property, and one of the clearest is as follows:-Permeability is the ability which a material has to transmit magnetizing force, and is expressed numerically as the ratio between the magnetic lines per unit area and the magnetizing force. Let B equal the lines per unit area and the permeability represented by then
If a column of air in a coil is given a magnetizing force equal to H, the lines per unit area or flux density will be the same, of in other words, air has • permeability of 1. It has been found by experiment that if a piece of iron is placed in the coil and subjected to the same force H, that the flux density B is much greater. For illustration; if the iron were given a magnetizing force that would produce 10 lines in the air, it would be found to contain 4070 lines per unit area. Then, by dividing the flux density of the iron by what it would be in the air the permeability is found to be 407. That is, the iron has the ability or capacity of carrying 407 more lines per unit area than the air at this density. All non-magnetic materials like paper, coton, brass, etc., are considered as having a permeability of 1.
Continued evperiments brought out the fact that the permeability of iron and steel varied much for different degrees of magnetization. It actually seems as though the . magnetic lines occupied space, and there soon comes a point at which the iron becomes saturated and any increase in the flux density requires more power in proportion than it did before saturation was reached. The result of these experiments have been ploted in curves with B for ordinates and H for abscissae. Fig. 5 shows approximately the curves for different irons and steels. These curves show very clearly the point of saturation to be where the curve turns and runs nearly horizontal. They also show that some varieties of steel and iron have a greater capacity than others. For example, soft annealed iron requires nearly three times the flux density that cast iron does to become saturated. The practical working limit of flux density B in good wrought iron is about 125,000 lines per square inch and in cast iron the working limit is reached at about 70,000 lines. However, the permeability of different pieces of the same kind of iron vary so that in extremely nice calculations it is necessary to test the piece to be used. In ordinary work, and especially in small dynamo designs, this degree of nicety is not necessary; there being other points in the design which would have more weight in their variation than this.
Joints in the iron of a magnet circuit seem to add a reluctance, or magnetic resistance, to the lines of force, requiring additional 31 M. F. to drive a given number of lines. The exact effects have to be determined experimentally and vary in proportion to the number of lines of force. For a low unit intensity (H-) the loss due to a joint amounts to about 20 per cent., while with a large density the loss runs as low as 2 per cent. The joint is really a very small air gap, and it is presumed that with the increased magnetic density the resulting attraction of the pieces of iron will reduce the air gap which would account for the decrease in loss. With very high magnetic density, of say 100,000 to 125,000 lines per sq. in., the attraction of one piece to the other at the joint will cause a pressure of approximately 200 lbs. per sq. in.
It is evident that an air gap in a non-magnetic circuit, or a space filled with non-magnetic material, which would have a permeability of 1 would require a considerable increase in M. M. F. to force the same number of lines as in the iron itself. The adjustment of air gaps in dynamos and motors, particularly small ones, needs much care both in designing and making the machines. If a large air gap is present, greater power is needed to force the required lines of force across it, as the evident power is drawn from the armature it is evident that its output is reduced and therefore the efficiency of the machine.
As stated before, some magnetic materials retain magnetism after the. source has been removed. Illustrations of this are steel, especially hard steel, and hardened irons. These have this property to a large degree. This remaining magnetism is termed Residual Magnetism. The ability of holding residual magnetism is not, however, confined to steel and hard iron. The softest iron will retain a slight amount. This property is a very fortunate one for dynamo designers, as it makes it possible for a dynamo to start generating current without an external application of magnetism to the fields every time the machine is started. This residual magnetism can be removed by reversing the magnetism with a coil of wire or by heating. Reversing the magnetism is uncertain as too strong a force in the opposite direction will produce residual in that direction. Heating iron to a dull or cherry red will remove all traces of the magnetism.