#2H.     Magnetic Fields -- History

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.

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.

Note:

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.
  1.     Lay the compass on a table, face upwards. Wait until it points north.
  2.     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.
  3.     Grab one end of the wire in one hand and press against one end of the battery.
  4.     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!

  5.     Repeat with the connections of the battery reversed. Note that the needle now swings 90 degrees in the opposite direction.

  6.     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.

Futher reading:

--"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.