On the Magnet, by William Gilbert

Chap. xv. The Magnetick Virtue which is conceived in Iron is more apparent in an iron rod than in a piece of iron that is round, square, or of other figure.

D uly was it said before that the longer magnet attracts the greater weight of iron167; so also in a longish piece of iron which has been touched the magnetick force conceived is stronger when the poles exist at the ends. For the magnetick forces which are driven from the whole in every part into the poles are not scattered but united in the narrow ends. In square and other angular figures the influence is dissipated, and does not proceed in straight lines or in convenient arcs. Suppose also an iron globe have the shape of the earth, yet for the same reasons it drags magnetick substances less; wherefore a small iron sphere, when excited, draws another piece of iron more sluggishly than an excited rod of equal weight.

167 Page 83, line 5. Page 83, line 5. magnes longior maiora pondera ferri attollit.— Gilbert discovered the advantage, for an equal mass of loadstone, of an elongated shape. It is now well known that the specific amount of magnetism retained by elongated forms exceeds that in a short piece of the same material subjected to equal magnetizing forces.


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