No honest man can be all things to all people.
In Chapters 4 to 6, we limited our discussions to static electric fields characterized by
E or D. We now focus our attention on static magnetic fields, which are characterized
by H or B. There are similarities and dissimilarities between electric and magnetic fields.
As E and D are related according to D = eE for linear material space, H and B are
related according to B =
Table 7.1 further shows the analogy between electric and
magnetic field quantities. Some of the magnetic field quantities will be introduced later
in this chapter, and others will be presented in the next. The analogy is presented here
to show that most of the equations we have derived for the electric fields may be readily
used to obtain corresponding equations for magnetic fields if the equivalent analo-
gous quantities are substituted. This way it does not appear as if we are learning new
A definite link between electric and magnetic fields was established by Oersted
1820. As we have noticed, an electrostatic field is produced by static or stationary charges.
If the charges are moving with constant velocity, a static magnetic (or magnetostatic) field
is produced. A magnetostatic field is produced by a constant current flow (or direct
current). This current flow may be due to magnetization currents as in permanent magnets,
electron-beam currents as in vacuum tubes, or conduction currents as in current-carrying
wires. In this chapter, we consider magnetic fields in free space due to direct current. Mag-
netostatic fields in material space are covered in Chapter 8.
Our study of magnetostatics is not a dispensable luxury but an indispensable necessity.
The development of the motors, transformers, microphones, compasses, telephone bell
ringers, television focusing controls, advertising displays, magnetically levitated high-
speed vehicles, memory stores, magnetic separators, and so on, involve magnetic phenom-
ena and play an important role in our everyday life.
Hans Christian Oersted (1777-1851), a Danish professor of physics, after 13 years of frustrating
efforts discovered that electricity could produce magnetism.
Various applications of magnetism can be found in J. K. Watson,
Applications of Magnetism.
York: John Wiley & Sons, 1980.