On the Magnet, by William Gilbert

Chap. xxii. That an armed Loadstone draws Iron no more than an unarmed one: And that an armed one is more strongly united to iron is shown by means of an armed loadstone and a polished cylinder of iron.


I f a cylinder be lying on a level surface, of too great a weight for an unarmed loadstone to lift, and (a piece of paper being interposed) if the pole of an armed loadstone be joined to the middle of it; if the cylinder were drawn from there by the loadstone, it would follow rolling; but if no medium were interposed, the cylinder would be drawn along firmly united with the armed loadstone, and in no wise rolling. But if the same loadstone be unarmed, it will draw the cylinder rolling with the same speed as the armed loadstone with the paper between or when it was wrapped in paper.

Armed loadstones of diverse weights, of the same ore vigour and form, cling and hang to pieces of iron of a convenient size and proportionate figure with an equal proportion of strength. The same is apparent in the case of unarmed stones. A suitable piece of iron being applied to the lower part of a loadstone, which is hanging from a magnetick body, excites its vigour, so that the loadstone hangs on more firmly. For a pendent loadstone clings more firmly to a magnetick body joined to it above with a hanging piece of iron added to it, than when lead or any other non-magnetick body is hung on.

A loadstone, whether armed or unarmed, joined by its proper pole to the pole of another loadstone, armed or unarmed, makes the loadstone raise a greater weight by the opposite end177. A piece of iron also applied to the pole of a magnet produces the same result, namely, that the other pole will carry a greater weight of iron; just as a loadstone with a piece of iron superposed on it (as in this figure) holds up a piece of iron below, which it cannot hold, if the upper one be removed. Magneticks in conjunction make one magnetick. Wherefore as the mass increases, the magnetick vigour is also augmented.

An armed loadstone, as well as an unarmed one, runs more readily to a larger piece of iron and combines more firmly with a larger piece than with a lesser one.

177 Page 90, line 9. Page 90, line 9. nempè vt alter polus maius pondus arripiat.— This acute observation is even now not as well known as it ought to be. Only so recently as 1861 Siemens patented the device of fastening a mass of iron to one end of an electromagnet in order to increase the power of the other end. The fact, so far as it relates to permanent magnets was known to Servington Savery. See Philos. Transactions, 1729, p. 295.


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