On Physical Lines of Force/Explainers

On Physical Lines of Force: map
Reading the edition Start here Notation and concordance Glossary Index of numbered equations Figures
Reading by level The story: Introduction Plain language: the explainers The modern equations, derived
Maxwell's paper Part I Part II Part III Part IV Sources and editorial method Chronology
Related Hamilton's principle Noether's theorems

On Physical Lines of Force → Explainers

Thirteen short pages covering each major idea in the paper, written for a general reader. No equations. Each answers one question and can be read on its own, though they follow a natural order.

If you read them straight through you will have the whole argument of the paper without meeting a single formula.

The problem

• What is a line of force? — the iron filings, and why Faraday thought they meant something
• Why Maxwell wanted a mechanism — what was unsatisfying about action at a distance, and why he chose stress over waves

The machine

• The sea of spinning cells — what Maxwell put in empty space, and why spinning things
• Why spinning makes a pull — how centrifugal force becomes magnetic attraction
• What is an electric current? — the gear-train problem, and the particles that solve it
• Why a changing magnet makes a current — induction as the inertia of flywheels

The two discoveries

• Why the cells had to be springy — a repair made for an unrelated reason
• The missing term — the addition that made radio possible
• Two ways to measure electricity — and why their ratio is a speed
• How Maxwell found the speed of light — two electrical measurements, one optical answer

The last part, and after

• Magnetism twists light — Faraday's discovery, and why it convinced Maxwell that magnetism is rotation
• A wrong model that produced right answers — what to make of a theory whose mechanism was abandoned
• What came after — 1865, 1873, and the verdict

If you only read one

The missing term. It is the shortest path to understanding why this paper matters, and it contains the single strangest fact in the history of physics: the term that makes wireless communication possible was added to fix a difficulty about imaginary spinning cells.

Where to go next

• The Introduction gives the same story at more length, with a little mathematics.
• The four Part sections — I, II, III, IV — follow Maxwell's own argument and quote him at length.
• The modern derivations take each result through to the equations as they are written today.

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