of the bar magnet the north pole, and the opposite end, the south pole. Many of us remember playing with magnets when we were children. It was fun to bring in a second bar magnet. It was noted that if its south pole was brought into the vicinity of the first magnet's south pole, the magnets physically repelled each other. On the contrary, when the north pole of one magnet was brought into the vicinity of the south pole of the other, they were attracted. Suppose the strength of the magnet in Figure A.9 is increased. The magnetic field will be strengthened in proportion, and is conventionally represented by a more congested arrangement of lines of magnetic induction. The force that will be exerted upon a pole of another magnet located at any point in the magnetic field will depend upon the intensity of the field at that point. This field intensity is represented by the notation H. In the preceding paragraphs we stated that the flux density B is the number of lines of magnetic induction passing through a unit area. By definition, unit flux density is one line of magnetic induction per square centimeter in air. Thus, in air, field intensity H and the flux density B have the same numerical value. A.6.2 Electromagnetism Any conductor where electrical current flows sets up a magnetic field. When the current ceases, the magnetic field collapses and disappears. If we form that wire conductor into a loop and connect each end to an emf source, current will flow and a magnetic field is set up. Let's say that the resulting magnetic field has an intensity H. Now take the loop and produce a second turn. The resulting magnetic field now has an intensity of 2H; with three turns, 3H, and SO forth. A.7 INDUCTANCE AND CAPACITANCE Figure A.10 A magnetic field surrounding a solenoid with an air-core. Figure A.10 shows a spool on which we can support the looped wire turns. Some will call this spool arrangement a solenoid. If a solenoid is very long when compared with its diameter, the field intensity in the air inside the solenoid is d