Experiment IV-10

Magnetic Fields

connecting the phenomena


Magnetism was known in ancient society.  One of the older stories, the Odyssey by Homer (~800 B.C.), describes Odysseus' journey home after war, including a voyage past a magnetic mountain capable of sucking the nails out of passing ships resulting in their demise.  While no mountain is sufficiently magnetic or likely any ship so poorly constructed, the story suggests that Homer was familiar with magnetism.  In medieval times there were tales of objects made magnetic when struck by lightning.

But magnetic phenomena remained little more than a novelty until a century before Marco Polo when tales perhaps from China described how floating magnets could be used as lodestones (journey stones) for travelers to find their way on cloudy days.  Soon mariners were using compasses showing the approximate direction of north to navigate under clouds obscuring the sun and stars and beyond sight of earthly landmarks.

GilbertIn 1600 William Gilbert (1544-1603 shown at right), physician to Queen Elizabeth I of England, wrote a book, de Magnete, distinguishing electric and magnetic effects.  Electric effects created by rubbing objects such as glass and pitches such as amber (and later rubber) could easily be wiped off while magnetic effects were permanently imbedded in natural minerals such as found in the prefecture of Magnesia along the central east coast of Greece.

Based on the Greek conclusion that the most perfect shape is a sphere, Gilbert suggested a piece of magnetic rock should be chiseled into a sphere.  Proposing that the Earth must be a giant lodestone, he instructed that each new magnet should be hung by a string, free to rotate.  Once rotation stopped, the side towards the north should be labelled as the new magnet's North seeking pole.  Gilbert noted that unlike electric objects, magnets always have two magnetic regions (poles).  Like poles (such as two north poles, or two of the opposite poles called south) repel each other while poles of opposite kinds attract.  (Note: If the magnetic pole attracted towards the Earth's arctic is to be labeled north, then what kind of magnetic pole must the Earth have in the Arctic?)  Gilbert speculated that the moon might also be a magnet caused to orbit by its magnetic attraction the Earth. (This was perhaps the first proposal that a force might cause a heavenly orbit.)

Gilbert's book led to much interest and experimentation with electric phenomena.  In 1800 Alesandro Volta's invention of the electric battery led to subsequent discoveries about moving electric charges called electric currents.

Humans rarely invent new ideas radically different from everything previous.  Most of the world's great ideas are modifications from previous ideas.  But not all ideas of scientist come from science.  Often philosophical or religious ideas lead to ideas which can be investigated by science.  An example comes from a group of German philosophers who built on the belief that there is but a single God, creator and governor of the universe.  Immanuel Kant (1724 - 1804) set out to determine the limits of human reasoning.  F.W.J.Schelling (1775 - 1854) developed Kant's ideas into Naturphilosophie which in part suggested that if there is but one cause for everything, that the various distinct forces discovered by scientists following the tradition of Isaac Newton must be a false dichotomy:  Instead all forces must somehow be just manifestations of a single cause.  (Nature Philosophy influenced people in many fields such as Kierkegaard, Marx, Nietzsche, Heidegger, and Einstein.)  Volta's invention of the electrical battery showed such a connection between chemical forces and electrical forces. And when others ran a strong electrical current through a small gage wire causing the wire to heat and glow, they found a connection between electricity, heat and light.

OerstedHans Christian Oersted (1777 - 1851 at left), as Professor of Nature Philosophy in Copenhagen, had long assumed a similar connection between electricity and magnetism.  He had planned to try the demonstrate such a connection during a course of lectures he was giving during the Winter of 1819-20, but having failed to test the demonstration before class, decided to postpone the demonstration until later.  He had previously placed a wire carrying electric current at right angles to a magnetic compass needle but found no effect.  But as time permitted near the end of the lecture and the possibility of success seemed more likely to him, Oersted placed a small wire carrying electric current instead parallel over a magnetic compass needle covered with glass.  This caused a small deflection.  Perhaps due to a well earned reputation for failed experiments and demonstrations, this historic discovery made little impression upon either Oersted or the audience!  But having more time during the Summer, Oersted repeated the experiment finding that the effect was surprisingly greater using a fatter wire which generated LESS heat and no light.  In July 1820 he publicly announced the discovery of the connection between electricity and magnetism stating that the magnetic effect of the electrical current has a circular motion round it, so that below the wire the magnetic needle is deflected to one side while the deflection is in the opposite direction above the wire.  Oersted experimentally verified that as the needle receives a magnetic force due to the current in the wire, the wire likewise receives an opposite magnetic push by the needle.  (Recall Newton's third law).

André-Marie Ampère (1775 - 1836) exactly one week after receiving news of Oersted's discovery showed at the September 18th meeting of the Academy in Paris that two parallel wires carrying electric currents magnetically attract each other if the currents are in the same direction and repel if the currents are in opposite directions.  Ampère leaped to a revolutionary conclusion that all magnetism is merely due to electricity in motion.  Ampère's friend Augustin Fresnel noted that substances which are magnetic such as iron might have electrical currents circulating around their molecules.

FaradayB field about wireThe English opponent to Ampère lacked the academic credentials expected of scientists!  Born to a poor family, apprenticed to a bookbinder as the trade negotiated for him, Michael Faraday (1791 - 1867 shown at right) educated himself by reading at night the books he was binding by day.  Enticed by the science he had read, Faraday attended free public lectures by Humphry Davy, laboratory director of the Royal Institution, took copious notes, and sent a copy of them in a letter in 1812 to Davy pleading for a new occupation.  Hired in the Spring of 1813 by the Royal Institution first as an assistant to wash dishes and clean after lecture-demonstrations by prominent scientists, Faraday became an able assistant to Davy, later replaced Davy upon Davy's retirement, and still later filled the chair of chemist created for him.  Lacking any understanding of mathematics and confused by much of what Ampère wrote, Faraday repeated for himself most of the new discoveries in electricity and magnetism and developed an intuitive vision for electric and magnetic phenomena, becoming the English expert in the fields.

Electric fieldFaraday envisioned lines of electric force originating on positive electric changes and terminating on negative electric charges.  These lines need not be straight, but are continuous following the path of least resistance, for example through a metal conductor in preference to through air.  A positive electric charge placed in an electric field would experience a force in the direction of the lines of force.  Negative charges would experience a force in the opposite direction.  Where lines of force are more concentrated, the force would be greater.

Magnetic field inside loopMagnetic lines of force are circular about any moving electric charge, lacking any ends.  (See diagram above.)  Magnetic dipoles would experience forces causing alignment when placed in magnetic fields.  If a wire conducting electric current is formed in the shape of a circular loop, a stronger magnetic field flux is produced inside the loop.  Additional loops forming a coil of wire increase the flux proportionally.

If a charge moves across to a magnetic field, the charge experiences a force (as does the field by Newton's third law) F = qv x B where the velocity and magnetic field strength B are vectors, and the force is the cross product at perpendicular to the plane generated by v and B.  (If you place your right hand with fingers pointing in the direction of the magnetic field and thumb in the direction a positive charge moves across the field, the force will be in the direction the palm of your hand pushes.)


Gilbert knew that two magnetic poles can attract or repel, depending on the kind of poles.  Ferromagnets as they are now called are the most magnetically active substances. Ferromagnets have atoms with permanent dipole moments, and when these materials form solids if the chemical bond lengths are right and neighboring atoms aligned with each other in arrangements with lower energy.  But often neighboring regions have random alignments making a very weak net magnetic field.  Pierre Curie (1859 - 1906) found that if a ferromagnetic material is heated to too high a temperature, now called the Curie temperature, the material ceases to be ferromagnetic.  At high temperature the thermal motion of the atoms is so violent that the electrons in the bonds are no longer able to keep the dipole moments aligned.  When the ferromagnetic material cools the magnetic regions reform.  If a ferromagnetic material is placed in a strong magnetic field, the regions can aligned with the external field.  When the external field is removed, the electrons in the bonds maintain the alignment and the magnetism remains.

Ordinary soft iron such as in common metal paper clips and pins is always attracted to a magnet, regardless of the pole of the magnet.  Gilbert guessed that near a permanent magnet the iron became a temporary magnet, creating a pole suitable for attraction.  The other end of the iron would temporarily becomes the opposite pole because magnetic poles always come in matched pairs.  When a paramagnetic material is placed in a strong magnetic field, it becomes a magnet as long as the strong magnetic field is present.  But when the strong magnetic field is removed, the net magnetic alignment is lost as the dipoles relax back to their normal random motion.  Paramagnetism is present in materials which have unpaired electrons.  Below the substance's Curie temperature a paramagnetic material becomes ferromagnetic.

Other materials, called diamagnetic, have atoms with no permanent dipole moment because all of their electrons exist only in magnetically opposing pairs.  When they are placed in a strong magnetic field, Lenz's law acts on the orbiting electrons and causes an atomic dipole moment to appear directed oppositely to the direction of the magnetic field. The effect produces very weak repulsion.


  1. Place a magnet near iron straight pins or paper clips.  Some will stick to the poles, but in addition, other pins often can stick to those pins.  When the pins are pulled loose, they become non-magnetic again.

  2. Using a small button shaped magnet, (not a flat refrigerator magnet which is too weak), and two straight pins, put the points of the pins right next to each other and note the attraction or repulsion of the other ends on the pins.  What must be the arrangement of magnetic poles that makes this happen?  (This is essentially a magnetic analog of an electroscope used to detect electric charge.)

  3. Try to find locations on the magnet to place the ends of the pins so that the other ends of the pins attract.  What must be the arrangement of magnetic poles that makes this happen?

  4. Using two flat refrigerator magnets, place them flat against each other the try to slide them apart by pulling on opposite ends.  Arrange the flat magnetic strips at 90° and repeat sliding them apart.  Often you can find an alignment where they will not smoothly slide apart.  Often these flat refrigerator magnets are formed of magnetic materials that have magnetic stripes with opposite poles.  From the sliding behavior you observed, determine what must be the arrangement of magnetic poles that makes this happen.

  5. If the refrigerator magnets do have stripes of opposite poles, determine what the alignment and spacing is for each.  Are the spacings the same on both?  Consider how the experimental results would be different if one had half the spacing and the other double.

  6. Consider placing a lot of loops along side each other forming a coil of wire.  Diagram what the shape of the magnetic field would be for a coil of wire.  What aspects of the field would be different compared to a single loop of wire carrying electric current.

  7. SSRL replacement magnetsHigh energy particle accelerators at the national laboratories use magnets for a variety of purposes.  Determine what arrangement of magnets would be needed to curve a beam of electrons in a clockwise (viewed from above) circular path.

  8. Magnets are used to focus beams of particles into smaller bundles.  Suggest what arrangement of magnets would be needed in pairs of quadapoles to focus a beam of protons.

  9. Sometimes investigators wish to improved the apparatus to investigate particles with higher energies.  Propose modifications to the magnets that could be used for working with electrons with ten times as much kinetic energy.  (At right are color coded magnets at the Stanford Linear Accelerator Center being readied for improving the SPEAR Synchrotron ring.)

  10. It is sometimes necessary to detect when a bundle of particles such as electrons passes by a certain location in a high energy particle accelerator.  Suggest a design that could use the magnetism of the passing electrons to detect their presence.

Record your observations and proposals in your journal.  If you need course credit, use the information in your journal to construct a formal report.



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created 4/22/2004
latest revision 4/30/2004
by D Trapp
Mac made