1600 Magnetism

Magnetism

• The Chinese were the first to create compasses made of lodestones
• and to use them for navigation.
• Western civilizations had only fantastical notions
• of why compasses worked
• until William Gilbert did some experiments
• and declared that the earth was a big magnet.
• Like any magnet, other magnets align with it, north and south.
• Moreover, he concluded, the earth at its core must be made of iron.

Experimental method

• William Gilbert found that lodestone is a form of iron
• and that iron itself can be made to act like a lodestone.
• He floated an iron wire by wrapping it around a cork
• and found that another piece of iron at a distance would move it.
• A bar of iron loses its magnetism when it is red hot
• and regains its power to attract as it cools.
• Because a rubbed amber does not disturb a candle flame,
• he realized electric attraction is not a disturbance of the air.
• Because heat and water affect electrical and magnetic objects differently
• he thought that they were distinct effects.
• He cut a magnet in half and noted that each half
• had its own north and south poles.
• He made himself a model of the earth,
• a spherical magnet that he named a terrella.
• The terrella mimicked the magnetic properties of the earth,
• its power to orient a compass to its north
• and to dip a compass below the horizon according to its lattitude.
• Thus William Gilbert was one of the first physicists
• to establish the value of experiment over scholastic teaching.

Action at a distance

• William Gilbert wrote, “For since no action
• can take place by means of matter unless by contact,
• these electricks are not seen to touch, but, as was necessary,
• something is sent from the one to the other,
• something which may touch closely
• and be the beginning of that incitement.”
• James Clerk Maxwell explained that electromagnetism
• is not action at a distance. Einstein explained that gravity
• is not action at a distance. Electromagnetism is only
• the influence of a localized field, and gravity is only
• a curvature of space and time. Magnetism
• is a field resulting from an electric current
• or the alignment of magnetic moments of many atomic nuclei.
• But what is an electromagnetic field?
• How does mass or energy bend space and time?
• Most of us have a murky understanding
• of why a magnet can form a pattern in iron filings.
• But we take it for a fact because we played with magnets
• when we were children, or we can test them now.