William Gilbert Aligns Several Loadstones
William Gilbert's De Magnete, Book Two, part 5
This post is part of a series of posts about William Gilbert’s De Magnete (On the Magnet1), which is composed of six books. This is the fifth and last post on Book Two. Here are the previous posts related to De Magnete.
De Magnete, Nothing Less than the First Ever Work of Experimental Physics
Book One, part 1: William Gilbert Writes about the Loadstone
Book One, part 2: William Gilbert Examines Iron, Calls Aristotle's Earth Element Dead
Book Two, part 1: William Gilbert Compares Electric Bodies to Magnetic Bodies
Book Two, part 2: William Gilbert Discusses Magnetic Bodies
Book Two, part 3: William Gilbert Considers the Internal Structure of the Terrella
Book Two, part 4: William Gilbert States that the Moon Causes the Tides
In this post, I will focus on interesting results that Gilbert presents with respect to armed loadstones and the interaction of multiple loadstones.
An armed loadstone is one in which a loadstone, preferably oblong-shaped, is capped with a thin layer of iron. It turns out that arming a loadstone increases the attraction of iron by the loadstone. Chaining multiple loadstones together, they mutually reinforce each other.
A CONCAVE hemisphere of thin iron, a finger’s width in diameter, is applied to the convex polar superficies of a loadstone and properly fastened; or an iron acorn-shaped ball rising from the base into an obtuse cone, hollowed out a little and fitted to the surface of the stone, is made fast to the pole. The iron must be the best (steel), smooth, polished, and even. Fitted with this contrivance, a loadstone that before lifted only 4 ounces of iron will now lift 12 ounces. But the greatest force of the co-operating or rather unified matter is seen when two loadstones fitted with these projections are so joined as mutually to attract and lift each other: thus may a weight of 20 ounces be lifted, though either stone unarmed would lift only 4 ounces. [p.137]
Nevertheless, the distance in which a loaded loadstone attracts iron is not more than for an unloaded one.
THAT an armed loadstone lifts a greater weight is evident to all; but iron is drawn from the same distance, or rather from a greater distance, to the loadstone when the stone is without the iron helmet. This is to be tried with two pieces of iron of the same weight and form at equal distance, or with one and the same needle, tested first with the armed then with the unarmed stone, at equal distances. [p.139]
The armed loadstone only increases its attraction of iron if there is direct contact. The attraction is not increased if, for example, a piece of paper separates an armed loadstone and a piece of iron.
IT has been shown above that an armed loadstone does not attract at a greater distance than an unarmed one, but that it lifts a greater quantity of iron, if it be in contact with the iron and continuous therewith. But put a leaf of paper between, and this intimate coherence is hindered, nor are objects of iron held together by the action of the loadstone. [p.140]
The next two experiments focus on the interaction between two loadstones.
In the first of these experiments, placing a smaller loadstone above a larger loadstone increases the power of the smaller loadstone. If, in addition, the south pole of the smaller loadstone is aligned with the north pole of the larger loadstone, an iron piece will stand vertical at the north pole of the smaller loadstone.
A strong, large loadstone increases the power of another loadstone, as also the power of iron. If, on the north pole of a loadstone, you place another loadstone, the north pole of the second becomes stronger, and a piece of iron clings like an arrow to the north pole a, and not at all to the south pole b. And the pole a, when it is in a right line above with the axis of both loadstones, they being joined according to the magnetic laws, raises the piece of iron to the perpendicular: this it cannot do if the larger loadstone be moved away, for its strength is insufficient. But as a ball of iron on the pole of the terrella raises the piece of iron to the perpendicular, so, at the side, the iron is not directed toward the centre, but stands oblique and sticks everywhere; for in the iron ball the pole is ever the point of contact with the terrella’s pole, and it is not constant, as it is in the smaller terrella. [pp.146-147]
The point of this experiment is to demonstrate that the magnetism of the earth contributes to the magnetism of a loadstone. In addition, this strengthening of a neighboring loadstone applies even if both loadstones are strong:
The parts of the earth, as of all magnetic bodies, are in accord and enjoy neighborhood with each other: there is in them all mutual love, undying good-will. The weaker loadstones are refreshed by the stronger ones, and the less vigorous bring no damage to the more vigorous. Yet a strong loadstone exerts more attraction in another strong one than in one that is feeble, for a vigorous stone contributes forceful action, and itself hastes, flies to the other, and solicits it vehemently; accordingly there is co-operation, and a clearer and stronger cohesion. [pp.147-148]
In the second of these experiments, a smaller loadstone is placed near a larger one, with the poles aligned. The attraction of an iron bar towards the south pole of the smaller loadstone is stronger than it would be if the smaller loadstone were spun around and the iron bar would be placed near the north pole of the smaller loadstone.
As a rod of iron pulls iron away from the terrella, so does a small loadstone or a smaller terrella albeit of less force. Here the iron bar C coalesces with the terrella A, and thus its force is enhanced and awakened magnetically both in the end in conjunction and also in the distal [far] end by reason of its contact with the terrella; the distal end furthermore receives energy from the loadstone B, and the pole D of this magnet also gains force by reason of its favorable position and the nearness of the pole E of the terrella. Hence many causes coöperate to make the bar C, attached to the loadstone B, cling more strongly to that than to the terrella A. The energy called forth in the bar, also the energy called forth in the loadstone B, and B’s native energy, all concur; therefore D is magnetically bound more strongly to C than E to C.
But if you turn the pole F to the iron C, then C does not cling to F as it did before to D; for, within the magnetic field, stones so arranged stand in an unnatural order: hence F does not get force from E. [pp.159-160]
If we consider the larger loadstone to be the earth, then we can understand that the attraction of a piece of iron by a loadstone will depend on how exactly the loadstone is placed with respect to the earth:
Magnetic bodies gain force from other magnetic bodies if they be arranged duly and according to their nature in neighborhood and within the sphere of influence; and hence, when a terrella is imposed on the earth or on another terrella in such way that the south pole looks toward the north pole, and north is turned away from north, the energy and forces of its poles are augmented. Hence the north pole of a terrella in such position lifts a heavier piece of iron than the south pole does if that be turned away. In like manner the south pole, gaining force from the earth or the larger terrella when it is duly placed as nature requires, attracts and holds heavier bars of iron. In the other portion of the terrestrial globe, toward the south, as also in the southern parts of the terrella, the case is reversed, for, there, the south pole of the terrella is strongest when distal, as is the north pole of the terrella when it faces the earth or terrella. The farther a place is from the equinoctial line, whether of the earth or of a terrella, the greater is seen to be the accession of force; but nigh the equator the difference is slight; at the equator it is null; at the poles it is greatest. [pp.164-166]
So now that the generalities about magnetic attraction, or coition, as Gilbert calls it, have been summarized, it is time to move on to Book Three, which focuses on the direction of a compass needle.
If you wish to donate to support my work, please use the Buy Me a Coffee app.
William Gilbert. De Magnete. Dover, New York, 1958. Translation by P. Fleury Mottelay of De Magnete, first published in 1600.