Permeability

. The quality of material, which permits of a given magnetizing force setting up a greater or le number of lines of force within it, is called its permeability. More accurately, the permeability is the ratio existing between the amount of magnetization and the magnetizing force which produces such magnetization. The permeability of a substance is usually represented by the Greek letter µ (pronounced mu ). The intensity of the magnetizing force is commonly symbolized by H, and since the permeability of air is always taken as unity, we may expre the intensity of magnetizing force by the number of lines of force per square centimeter which it sets up in air. Now, if the space on which the given magnetizing force H were acting were filled with iron instead of air, then, owing to the greater permeability of iron, there would be set up a very much greater number of lines of force per square centimeter, and this number of lines of force per square centimeter in the iron is the measure of the magnetization produced and is commonly expre ed by the letter B . From this we have µ = B ÷ H Thus, when we say that the permeability of a given specimen of wrought iron under given conditions is 2,000, we mean that 2,000 times as many lines of force would be induced in a unit cro -section of this sample as would be induced by the same magnetizing force in a corresponding unit cro -section of air. Evidently for air B = H , hence µ becomes unity. The permeability of air is always a constant. This means that whether the magnetic density of the lines of force through the air be great or small the number of lines will always be proportional to the magnetizing force. Unfortunately for easy calculations in electromagnetic work, however, this is not true of the permeability of iron. For small magnetic densities the permeability is very great, but for large densities, that is, under conditions where the number of lines of force existing in the iron is great, the permeability becomes smaller, and an increase in the magnetizing force does not produce a corresponding increase in the total flux through the iron.