1600: Magnetism - The book of science

Magnetism
Experimental method
Action at a distance
Commentary

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.

William Gilbert was a physician, serving as the president of the Royal College of Physicians and as the personal physician of Elizabeth I and, later, James I.

Gilbert invented the first electrical measuring instrument, the electroscope. Our word electricity derives from his use of the Greek word for amber, elektron, to stand for its attractive property.

Today, particle physicists model an electromagnetic field as an exchange of virtual photons. It is one of the four fundamental physical interactions—electromagnetic, gravitational, strong nuclear, and weak nuclear.