The Physics
Opus in profectus


search icon



Outline of the story told historically. Basic ideas that even young children now know. Keep everything to an introductory level.

First comes Thales of Miletus (635–543 BCE) Greece (Ionia). Miletus is now on the western coast of Turkey in what was then a region of Greece known as Ionia (source of the chemical term ion, but that's another story).

A nice quote from Thales would be nice here.

Iron filings clinging to a rock

Some minerals such as magnetite (Fe3O4) are obviously magnetic.

Chinese navigators knew that magnetic rocks align themselves north-south — the south-pointing spoon (指南針, zhǐnán zhēn, pointing south needle; 指南魚, zhǐnán yú, pointing south fish, guidefish; 羅盤 luópán, box and needle?).

若遇天景曀霾,夜色暝黑,又不辨方向,則當縱老馬前行,必識道路。 或出指南車及指南魚以辨所向。指南車法,世不傳。魚法,用薄鐵葉剪裁,長二寸,闊五分,首尾銳如魚形,置炭火中燒之,候通赤,以鐵鈐鈐魚首,出火,以尾正對子位,蘸水盆中,沒尾數分則止,以密器收之。用時置水椀於無風處,平放魚在水面令浮,其首常南向午也。  When troops encountered gloomy weather or dark nights, and the directions of space could not be distinguished, they let an old horse go on before to lead them, or else they made use of the south-pointing carriage, or the south-pointing fish (chih nan yu 指南魚) to identify the directions. Now the carriage method has not been handed down, but in the fish method a thin leaf of iron is cut into the shape of a fish two inches long and half an inch broad, having a pointed head and tail. This is then healed in a charcoal fire, and when it has become thoroughly red-hot, it is taken out by the head with iron tongs and placed so that its tail points due north (lit. in the tzu direction). In this position it is quenched (chan shu 蘸水) with water in a basin, so that its tail is submerged for several tenths of an inch. It is then kept in a tightly closedbox (i mi chhi shou chih 以密器收之). To use it, a small bowl filled with water is set up in a windless place, and the fish is laid as flat as possible upon the water-surface so that it floats, whereupon its head will point south (lit. in the wu direction).
曾公亮。1044年   Zeng Gongliang, 1044, Translated by Joseph Needham, 1962 (paid link)


The compass in real sense was created by a Chinese geomantic omen master in late Tang Dynasty, who originally used it for divination.

Find something historical.

A spiinning globe covered in evenly spaced compasses

The north magnetic pole of a compass points in the general direction of the north geographic pole of the Earth. Since opposite magnetic poles attract, this means that the south magnetic pole of the Earth is somewhere near its north geographic pole.

Next comes Peter Peregrinus (as he is known in English) a.k.a. Pierre Pèlerin de Maricourt (presumably his proper french name) a.k.a. Petrus Peregrinus de Maharncuria (his latin title, which means "Peter the Pilgrim of Maricourt"). Peter wrote what is commonly known as the Epistole de Magnete or Letter on the Magnet. It's full title is Epistola Petri Peregrini de Maricourt ad Sygerum de Foucaucourt, militem, de magnete (Letter on the Magnet of Peter Peregrinus of Maricourt to Sygerus of Foucaucourt, Soldier"). It was written on 8 August 1269 during the siege of the city of Lucera — the last remaining stronghold of Islam on the "calf muscle" of the boot-shaped peninsula that is now called Italy.

Peter's work was so complete that no further studies on the properties of magnets were done until the monumental work of William Gilbert in 1600 — Tractatus sive Physiologia Nova de Magnete, Magneticisque Corporibus, et Magno Magnete Tellure (On the Magnet, Magnetic Bodies, and the Great Magnet of the Earth). De Magnete was the text in which Gilbert revealed the results of his research on magnetism and attempted to explain the nature of magnets and the five motions associated with magnetic phenomena. The work met with great acclaim and was republished in 1628 and 1633.

William Gilbert (1544–1603) England

Find something.


magnetic elements

Photograph of a crane lifting scrap metal
A lifting magnet in action — Brooklyn, New York

More on ferromagnetism later

The Earth is a magnet, geomagnetism, poles: north seeking and south seeking

rule of action: opposite poles attract, like poles repel

types of magnets

magnetic field

informal definition: compare to the other fields

A comparison of force fields * Most emphatically, No! The magnetic field breaks this trend.
phenomenon origin field symbol
gravity force due to mass force per mass g
electricity force due to charge force per charge E
magnetism force due to poles?* force per pole?* B

the real definition appears later

dilemma: breaking a magnet, there is no magnetic monopole, even down on the atomic scale there is no magnetic monopole, field lines heal themselves

Still, even though we don't have a formal definition of the magnetic field there's no reason why an informal definition can't suite us for a while.

properties of magnetic field lines

The symbol for the magnetic field is B (boldface) when describing the full vector quantity and B (italic) when describing the magnitude alone.

The SI unit of the magnetic field is the tesla [T], named in honor of the Serbian-American electrical engineer Nikola Tesla (1856–1943) born in a part of the Austro-Hungarian or Hapsburg Empire that is now the independent nation of Croatia. Tesla was a pioneer in the associated disciplines of alternating electric current and rotating magnetic fields. His basic designs for electric motors, generators and transformers in the early part of the 20th century were little changed by the beginning of the 21st century.

Another unit in common usage is the gauss [G], named in honor of the German mathematician Carl Friedrich Gauss (1777–1855). Gauss is generally regarded as the greatest mathematician of all time. Of particular interest to physicists were Gauss' work on curved surfaces, which were important in the fields of electrostatics and general relativity (all puns intended). The gauss is also a unit in the cgs system that was originally developed by Gauss and is sometimes also known as the Gaussian system.

Each of these units will be defined formally in a later section of this book. Right now I will tell you that the tesla is the bigger unit.

1 T =  10,000 G

The tesla is in fact too big for most practical purposes. As such it is usually divided into microtesla [μT] or nanotesla [nT]. The gauss is also a bit too large, but not as bad as the tesla, so milligauss [mG] and microgauss [μG] are more common.

1 T =  10 kG
1 mT =  10 G
1 μT =  10 mG
1 nT =  10 μG

The following table lists the magnetic field strength for various devices, events, or phenomena. Whenever possible a location was also specified. Like the gravitational and electric fields, the magnetic field grows smaller with increasing distance form the source.

Selected magnetic field values (largest to smallest)
B (T) device, event, phenomenon, process
~1013 neutron star, theoretical upper limit
1010~1011 neutron star, magnetar
108~109 neutron star, radio pulsar
1,000 strongest laboratory magnet, ephemeral
100 white dwarf star
45 strongest laboratory magnet, sustained
16 strong enough to levitate frogs
20 strongest superconducting magnet
2.4 strongest permanent magnet
1–4 MRI
1 strong laboratory magnet
0.45 large sunspot
0.15 iron bar magnet, at poles
0.10 refrigerator magnet
0.001 Sun, at poles
400 × 10−6 Jupiter, surface mean
100 × 10−6 Sun, surface mean
60 × 10−6 Earth, at poles
45 × 10−6 Earth, surface mean
30 × 10−6 Earth, at equator
10 × 10−6 AM radio broadcast at receiver
1 × 10−6 solar radiation on Earth's surface
180 × 10−9 100 W light bulb at 1 m, peak
150 × 10−9 Mercury, surface mean
150 × 10−9 Earth, altitude of geosynchronous orbit
50 × 10−9 Earth, magnetosphere nose
35 × 10−9 Moon, surface
5 × 10−9 interplanetary space near Earth
1 × 10−9 Earth, magnetosphere tail
500 × 10−12 interstellar space
100 × 10−12 intergalactic space
50 × 10−12 human heart
100 × 10−15 human brain


This section is intended to be a discussion of magnetism on the small scale, not just the magnetism of iron, nickel, and cobalt (although that will be its primary focus). Perhaps it should be titled "micromagnetism".

Everything's due to electron spin. Well, almost everything.

Types of magnetic behavior (in order of decreasing strength, more or less)
type spin alignment examples
ferromagnetic all spins align parallel to one another iron, cobalt, nickel, gadolinium, dysprosium, heusler alloys
ferri­magnetic most spins parallel to one another, some spins antiparallel magnetite (Fe3O4), yttrium iron garnet (YIG)
antiferro­magnetic periodic parallel-antiparallel spin distribution chromium, FeMn, NiO
paramagnetic spins tend to align parallel to an external magnetic field oxygen, sodium, aluminum, calcium, uranium
diamagnetic spins tend to align antiparallel to an external magnetic field nitrogen, copper, silver, gold, water, organic compounds
superdia­magnetic all spins align antiparallel to an external field meissner effect in superconductors



Basic types of steel (in order of increasing carbon content)
type composition magnetic? other characteristics
ferritic stainless Fe, Cr, C yes moderate corrosion resistance,
moderate durability
austenitic stainless Fe, Cr, Ni, C no high corrosion resistance,
moderate durability
martensitic stainless Fe, Cr, C yes moderate corrosion resistance,
high durability
non-stainless (high carbon) Fe, C yes low corrosion resistance,
high durability

Alloys made expressly for permanent magnets…

Ferromagnetic alloys made entirely of nonferrous metals…

magnetic recording

the basic mechanism


media formats

media shapes


ferromagnetic material

Common materials used for magnetic tape
type bias material comments
I normal gamma ferric oxide
first commercially manufactured in 1937
II high chromium dioxide
later replaced by layers of ferric oxide (Fe2O3) and cobalt (Co) with similar magnetic characteristics
III ferric chrome
quickly became obsolete
IV metal finely ground metallic iron later replaced by mixtures of finely ground iron and cobalt
n/a barium ferrite
magnetic stripes on bank and credit cards, high coercivity, less susceptible to accidental erasure

transition temperatures

The Curie temperature is named for the French physicist Pierre Curie (1859–1906), who discovered the laws that relate some magnetic properties to change in temperature in 1895.

The antiferro­magnetic equivalent of the Curie Temperature is called the Néel Temperature in honor of the French physicist Louis Néel (1904–2000), who successfully explained antiferromagnetism in 1936.

Curie temperatures of selected ferromagnetic materials
elements TC (K)
iron 1043
cobalt 1404
nickel 628
gadolinium 289
erbium 32
dysprosium 155
ferrous compounds TC (K)
barium ferrite 720
strontium ferrite 720
Alnico 1160
Alumel 436
Mutamel 659
Permalloy 869
Trafoperm 1027
NdFeB 580
SmCo5 990
Sm2Co17 1070
nonferrous compounds TC (K)
CrO2 390
CuAlMn3 ?
LaxCa1−xB6 900
MnAs 318
MnBi 633
MnSb 587
polymerized C60 ~500
Néel temperatures of selected antiferromagnetic materials
material TN (K)
CoCl2 25
CoF2 38
CoO 291
chromium 475
Cr2O3 307
erbium 80
FeCl2 70
FeF2 79–90
FeO 198
FeMn 490
α-Fe2O3 953
MnF2 72–75
MnO 122
MnSe 173
MnTe 310–323
NiCl2 50
NiF2 78–83
NiFeO 180
NiO 533–650
TiCl3 100
UCu5 15
V2O3 170

animal magnetism (magnetotaxis?)

health and safety


Magnetic field exposure when using various devices
device B (μT)
color tv/computer crt display 500
electric stove 1000
hair dryer 1000
maglev train 100


Average daily exposure to magnetic fields
location median (μT) range (μT)
earth's surface 45 40–60
workplace: clerical worker w/o computer 0.05 0.02–0.20
  clerical worker w/computer 0.12 0.05–0.45
  machinist 0.19 0.06–2.76
  electrical line worker 0.25 0.05–3.48
  electrician 0.54 0.08–3.40
  welder 0.82 0.17–9.60
home: typical US home 0.09 0.03–0.37


magnetic resonance imaging (nuclear magnetic resonance)