Page 59 - Teach Yourself Electricity and Electronics

Magnetic units 39

2-12 Magnetic flux lines around a coil of wire. The fines converge at the magnetic
poles.

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 cause
the lines of flux to be much closer together inside the material. This is the principle by
which 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 run
inward toward the magnetic south pole. But this is just a semantical thing, about which
theoretical physicists might speak. It doesn’t need to concern you for ordinary electri-
cal and electronics applications.

Magnetic units

The size of a magnetic field is measured in units called webers, abbreviated Wb. One
weber is mathematically equivalent to one volt-second. For weaker magnetic fields, a
smaller unit, called the maxwell, is sometimes used. One maxwell is equal to
0.00000001 (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 per
square meter or per square centimeter. A flux density of one weber per square meter
2
(1 Wb/m ) is called one tesla. One gauss is equal to 0.0001 weber, or one maxwell per
square centimeter.