Magnets are not the only source of magnetic fields.
In 1820, Hans Christian Oersted discovered that an electric
current flowing through a wire caused a nearby compass to deflect.
This indicated that the current in the wire was generating a
magnetic field. Oersted studied the nature of the magnetic field
around the long straight wire. He found that the magnetic field
existed in circular form around the wire and that the intensity
of the field was directly proportional to the amount of current
carried by the wire. He also found that the strength of the
field was strongest next to the wire and diminished with distance
from the conductor until it could no longer be detected. In
most conductors, the magnetic field exists only as long as the
current is flowing (i.e. an electrical charge is in motion).
However, in ferromagnetic materials the electric current will
cause some or all of the magnetic domains to align and a residual
magnetic field will remain.
also noticed that the direction of the magnetic field was dependent
on the direction of the electrical current in the wire. A three-dimensional
representation of the magnetic field is shown below. There is
a simple rule for remembering the direction of the magnetic
field around a conductor. It is called the right-hand clasp rule.
If a person grasps a conductor in one's right hand with the thumb
pointing in the direction of the current, the fingers will circle
the conductor in the direction of the magnetic field.
A word of caution about
the right-hand clasp rule
For the right-hand rule to work, one important thing that must be
remembered about the direction of current flow. Standard convention
has current flowing from the positive terminal to the negative
terminal. This convention is credited to Benjamin Franklin who
theorized that electric current was due to a positive charge
moving from the positive terminal to the negative terminal.
However, it was later discovered that it is the movement of
the negatively charged electron that is responsible for electrical
current. Rather than changing several centuries of theory and
equations, Franklin's convention is still used today.