Until 1820, the only magnetism known was that of iron
magnets and of "lodestones", natural magnets of iron-rich ore.
Edmond Halley
It was believed that the inside of the Earth was magnetized
in the same fashion, and scientists were greatly puzzled when they found that
the direction of the compass needle at any place slowly shifted, decade by
decade, suggesting a slow variation of the Earth's magnetic field.
How can an iron magnet produce such changes? Edmond Halley
(of comet fame) ingeniously proposed that the Earth contained a number of
spherical shells, one inside the other, each magnetized differently, each
slowly rotating in relation to the others.
Hans Christian Oersted was a professor of science at
Copenhagen University. In 1820 he arranged in his home a science demonstration
to friends and students. He planned to demonstrate the heating of a wire by an
electric current, and also to carry out demonstrations of magnetism, for which
he provided a compass needle mounted on a wooden stand.
Oersted's Experiment
While performing his electric demonstration, Oersted noted
to his surprise that every time the electric current was switched on, the
compass needle moved. He kept quiet and finished the demonstrations, but in the
months that followed worked hard trying to make sense out of the new
phenomenon.
What Oersted saw...
But he couldn't! The needle was neither attracted to the
wire nor repelled from it. Instead, it tended to stand at right angles (see
drawing below). In the end he published his findings (in Latin!) without any
explanation.
André-Marie Ampére in France felt that if a current in a
wire exerted a magnetic force on a compass needle, two such wires also should
interact magnetically. In a series of ingenious experiments he showed that this
interaction was simple and fundamental--parallel (straight) currents attract,
anti-parallel currents repel. The force between two long straight parallel
currents was inversely proportional to the distance between them and
proportional to the intensity of the current flowing in each.
[Only for those pursuing the math: this is not the basic
force formula. Given two short parallel currents I1 and I2, flowing in wire
segements of length L1 and L1 and separated by a distance R, the basic formula
gives the force between them as proportional to
I1 I2 L1 L1/R2
(it gets further complicated if the currents flow in
directions inclined to each other by some angle). To find then the force
between wires of complicated shape that carry electrical currents, all these
little bitty contributions to the force must be added up. For two straight
wires, the final result is as above--a force inversely proportional to R, not
to R2]
Maxwell
There thus existed two kinds of forces associated with
electricity--electric and magnetic. In 1864 James Clerk Maxwell demonstrated a
subtle connection between the two types of force, unexpectedly involving the
velocity of light. From this connection sprang the idea that light was an
electric phenomenon, the discovery of radio waves, the theory of relativity and
a great deal of present-day physics.
Repeat Oersted's
Experiment !
Note:
This experiment can also be performed on top of a projector
in the classroom, for the entire class to watch. See Teaching about Magnetism.
You will need:
A pocket compass.
A one-foot (30 cm)
length of fairly thick wire, insulated or bare.
A 1.5 volt
electric cell ("battery") of size "D" or "C". The
voltage is too low to cause any risk.
Lay the compass on
a table, face upwards. Wait until it points north.
Lay the middle of
the wire above the compass needle, also in the north-south direction (compare
to the above image "What Oersted Saw"). Bend the ends of the wire so
that they are close to each other.
Grab one end of
the wire in one hand and press against one end of the battery.
Grab the other end
with your other hand, and press momentarily against the other terminal of the
battery. The needle will swing strongly by 90 degrees.
Quickly disconnect
(it is not good for the battery to draw such a large current). The needle will
swing back to the north-south direction. Note that no iron is involved in
producing the magnetic effect!
Repeat with the
connections of the battery reversed. Note that the needle now swings 90 degrees
in the opposite direction.
Take a piece of
paper 2"x4" (5x10 centimeters) and fold the longer side into pleats,
about 3/8" (1 centimeter) high. Put the wire on the table, its middle in
the north south direction, put the pleated paper above it so that the wire is
below one of the pleats, and place the compass on top of the pleats. (Or else,
use a small block of wood, with a groove cut in its bottom for the wire.)
You can now repeat
the experiment with the compass above the wire (if two people perform the
experiment, they need no pleats or table--one can old the compass, the other
the wire and battery). Note that the needle swings in the opposite direction
than when the compass was below the wire.
Questions from Users:
*** On building an electromagnet
Futher reading:
--look up in an encyclopaedia "Halley, Edmond",
"Oersted, Hans Christian", "Ampere, Andre-Marie" and
"Maxwell, James Clerk."
--"From Falling Bodies to Radio Waves" by Emilio
Segre, W.H. Freeman and Co., 1984, gives a very good account of the history of
electricity and magnetism (and of physics up to 1895). Segre, who won the Nobel
prize in physics, wrote in a clear style with many insights and anecdotes about
the discoveries which laid the foundations of physics.
--"Oersted and the Discovery of Electromagnetism"
by Bern Dibner (Blaisdell Publ. Co., 1962), a slim book with details about
Oersted and his time.
--"Andre-Marie Ampere" by L.Pearce Williams,
Scientific American January 1989, p. 90.
--"Edmond Halley, Geophysicist" by Michael E.
Evans, Physics Today, February 1988, p. 41-45.
