Art. 1. Of letters as filings
Every letter on this page is a small piece of soft iron. Left alone, the letters lie flat in their lines and read as ordinary type. Bring a magnet near and each one becomes a weak magnet of its own, induced by the field around it, and turns to lie along the direction of that field.
Art. 2. Of the bar
The bar is treated as two poles of equal strength and opposite sign, after Gilbert. The pole strength is the moment divided by the distance between the poles. At any point on the page the field is the sum of the two contributions, and each falls off with the square of the distance.
B = (μ₀/4π) q r / r³
The faint lines drawn under the text are traced from the north pole along the field until they reach a south pole or leave the page.
Art. 3. Of piles and clinging
Where the field is weak the letters only turn. Above six gauss they begin to stand up from the paper, and above ten gauss they slide toward the nearer pole, so the type gathers in piles at each end of the bar. Above forty gauss a letter breaks out of its line and clings to the bar. It travels with the magnet until the field at its home falls below three gauss, and then it drops and drifts back into place.
Art. 4. Of scale
The field of the Earth at the surface is about half a gauss. A small alnico bar with a moment of two ampere square metres gives close to two hundred gauss one centimetre from a pole. One tesla is ten thousand gauss.
Art. 5. Of two magnets
A second bar adds its own poles to the sum. Like poles repel and unlike poles attract, and a free bar slides across the paper once the force between them is greater than friction. The lines crowd together in the gap where the bars attract and bend away from it where they repel.
Art. 6. To restore the page
Demagnetize. The bars lose their strength for a moment and every letter returns to its line.