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Why does a compass point north? Today the answer sounds obvious: the Earth behaves like a magnet. In 1600 it was not obvious at all, and many people explained the compass with stories about the pole star or magnetic mountains. William Gilbert settled it with a model and a series of experiments. This post tells his story, explains the physics, and works a bar-magnet problem of the kind you will meet in Class 12.
Who was William Gilbert?
Gilbert lived from 1544 to 1603 and came from Colchester in England. A review of his book by geophysicists Stuart Malin and David Barraclough, published in Eos in 2000 and archived by NASA, says he studied at St John's College, Cambridge, settled in London in 1573 to practise medicine, and became physician to Queen Elizabeth I in 1601. Magnetism was his side project, which he worked on from about 1581 to 1600, often with friends who met at his house.
What is De Magnete about?
His book, De Magnete, appeared in 1600 in Latin. The Eos review describes 115 chapters in six books, laid out much like a modern thesis: a survey of earlier work, experiments, discussion and open questions. An English translation by P. Fleury Mottelay is free on Project Gutenberg.
Three ideas stand out:
The Earth is a magnet. Book 1 ends with this claim, and Books 4 and 5 deal with how the compass needle varies from true north and how it tilts downward, which is called dip.
The terrella. Gilbert shaped lodestone into a sphere, a "little Earth", and moved small needles over it. He compared how they pointed and tilted with what compass and dip needles do on the real Earth, and found the same pattern.
Electric is not magnetic. He showed that rubbed amber attracts light objects in a different way from a magnet, and named that force electric, from the Greek word for amber.
He also tested folklore and dropped what failed. The review notes that he dismissed as "fable and falsehood" the idea that garlic weakens a lodestone.
Why was the terrella such a strong argument?
A model works when it reproduces several features at once. A needle over a magnetised sphere points towards one pole and tilts more as you approach a pole. If the Earth were a large magnet, a compass on Earth should do both, and it does. Gilbert's argument rested on matching behaviour, not on authority.
The field outside a uniformly magnetised sphere has the same shape as the field of a small bar magnet at its centre, a magnetic dipole. That is why your textbook can say the Earth behaves roughly like a giant bar magnet. The NCERT Class 12 chapter Magnetism and Matter opens with exactly this: the Earth behaves as a magnet with its field pointing approximately from geographic south to north. In the current rationalised edition, the detailed section on the Earth's magnetism (dip and declination) is not included, but bar magnets and dipole fields are.
Worked example: field of a bar magnet on its axis and its equator
A short bar magnet has magnetic moment m = 0.40 A m². Find the field at 0.20 m from its centre (a) on its axis, and (b) on its equatorial line. Use μ0/4π = 10⁻⁷ T m A⁻¹.
r³ = (0.20)³ = 0.008 m³.
Axial: B = (μ0/4π) × 2m/r³ = 10⁻⁷ × 0.80/0.008 = 10⁻⁷ × 100 = 1.0 × 10⁻⁵ T, pointing along the magnet's moment.
Equatorial: B = (μ0/4π) × m/r³ = 0.5 × 10⁻⁵ T, pointing opposite to the moment.
The axial field is twice the equatorial field at the same distance, and the directions differ. On a dipole-like Earth, this is why a needle lies nearly flat near the magnetic equator and tilts towards the vertical near the magnetic poles. For more practice, see our post on the magnetic dipole and the bar magnet.
What did Gilbert get wrong?
The last book of De Magnete links magnetism to the Earth's daily rotation and the motions of the planets. That part was wrong. Still, the review notes that the idea of a force acting at a distance between bodies fed into later thinking about gravity by Hooke and Newton.
What can a student learn from Gilbert's way of working?
Build a small model of a big thing. A terrella fits in your hand; the Earth does not.
Check one idea several ways. Direction and dip together made his case far stronger than either alone.
Test claims, even popular ones. The garlic test is a reminder to ask "has anyone tried this?"
Separate look-alike effects. Amber and lodestone both attract, but for different reasons.
Field lines are easier to understand when you can move a compass around and watch them. The Fermi Simulation Lab has interactive simulations for ideas that are easier to see than read. To see how Faraday later turned field lines into a working tool, read our post on Faraday and field lines.
Try this: put a compass at several points around a bar magnet, sketch the needle direction at each, and compare your sketch with the dipole field pattern in your textbook.
