=Permeability.=--Permeability is a measure of the ease with which magnetism passes through any substance. It is defined as: _the ratio between the number of lines of force per unit area passing through a magnetizable substance, and the magnetizing force which produces them_. [Illustration: FIGS. 120 and 121.--Illustrating the effect of introducing an iron core into a solenoid. In the upper figure, the air space or “air core” surrounded by the solenoid offers considerable resistance to the passage of magnetic lines, allowing only a small number to pass through. If a piece of iron be introduced, as in the lower figure, the number of lines will be greatly increased. The number of lines B passing through a unit cross section of the iron core divided by the number of lines H, passing through a unit cross section of the air core is called the _permeability_ and designated by the Greek letter μ.] In other words, it is the ratio of flux density to magnetizing force. Permeability is a measure of the ease with which magnetism passes through any substance. The permeability of good soft wrought iron is sometimes 3000 times that of air, varying with the quality of the iron. =Ques. What is the effect of increasing the magnetization?= Ans. The magnetic permeability decreases as the magnetization increases. =Ques. What is magnetic saturation?= Ans. The state of a magnet which has reached the highest degree of magnetization. [Illustration: FIG. 122.--Action of currents on solenoids. To demonstrate this fact experimentally, a solenoid is constructed as shown, so that it can be suspended by two pivots in the cups _a_ and _c_. The solenoid is then movable about a vertical axis, and if a rectilinear current QP be passed beneath it, which at the same time traverses the wires of the solenoid, the latter is seen to turn and set at right angles to the lower current; that is, in such a position that its circuits are parallel to the fixed current; moreover, the current in the lower part of each of the circuits is in the same direction as in the rectilinear wire. If, instead of passing a rectilinear current below the solenoid, it be passed vertically on the side, an attraction or repulsion will take place, according as the two currents in the vertical wire, and in the nearest part of the solenoid, are in the same or in contrary directions.] A magnet, just after being magnetized, will appear to have a higher degree of magnetism than it is able to retain permanently; that is, it will appear to be super-saturated, since it will support a greater weight immediately after being magnetized than it will after its armature has been once removed. For all practical purposes, magnetic saturation may be defined as: That point of magnetization where _a very large increase in the magnetizing force does not produce any perceptible increase in the magnetization_.