magnetized. Such substances are called ferromagnetic. A core of this kind cannot actually increase the total quantity of magnetism in and around a coil, but it will causethe lines of flux to be much closer together inside the material. This is the principle bywhich an electromagnet works. It also makes possible the operation of electrical trans-formers for utility current.Magnetic lines of flux are said to emerge from the magnetic north pole, and to runinward toward the magnetic south pole. But this is just a semantical thing, about whichtheoretical physicists might speak. It doesn’t need to concern you for ordinary electri-cal and electronics applications.Magnetic unitsThe size of a magnetic field is measured in units called webers, abbreviated Wb. Oneweber is mathematically equivalent to one volt-second. For weaker magnetic fields, asmaller unit, called the maxwell, is sometimes used. One maxwell is equal to0.00000001 (one hundred-millionth) of a weber, or 0.01 microvolt-second.The flux density of a magnetic field is given in terms of webers or maxwells persquare meter or per square centimeter. A flux density of one weber per square meter (1 Wb/m2) is called one tesla. One gauss is equal to 0.0001 weber, or one maxwell persquare centimeter.Magnetic units392-12Magnetic flux lines around a coil of wire. The fines converge at the magneticpoles.