The loadstone is frequently combined with substances which have small or no attractive powers ; and the poles, or points of contrary nature with respect to attractions and repulsions, being irregularly distributed, counteract in part one another's effects : thus a mass of loadstone often exhibits a comparatively small adhesive force ; and it happens at times that, when the mass is broken in pieces, some of the portions, containing a considerable quantity of the attractive material favourably disposed, exercise a greater power than was exhibited by the entire mass. It is found that a piece of loadstone will seldom suspend more than 5 or 10 times its own weight, though some small specimens have been obtained which are said to have suspended more than 200 times their weight.
If a piece of iron be placed in contact with either pole of a magnetised bar, the force of the coherence is that which is due to the direct action of that pole only on the contrary magnetism of the iron : the intensity is at least doubled when the substance to be suspended can be placed at once in contact with both poles ; and this is frequently effected by giving to the magnetised bar the form of a horse-shoe, so that the opposite poles may be near one another : but the must effectual method of increasing the power of a magnetic body is that of arming it.
Preparatory to arming a piece of loadstone the situation of its axis, that is, of a line joining its poles, must be determined by trial : the loadstone is then reduced to the form of a cube or parallelopiped nearly, having two opposite faces perpendicular to the axis ; and these faces are ground very smooth. Two plates of soft iron equal in length and breadth to these sides are applied to the latter so as quite to cover them, and are connected together by a bar of soft iron which is attached to them at the upper end of each (supposing the two plates to be in vertical positions): to this bar is affixed a hook or ring by which the whole apparatus may be suspended.
The inferior extremity of each plate is formed with a foot which projects under the loadstone, so that the latter may rest upon both ; and horizontal bands of copper or silver usually surround the loadstone and side pieces in order to keep the whole together. Under the feet is applied a bar or plate of iron, touching both ; and this is provided with a ring or hook from which is to be suspended the weight to be held by the attractive force of the loadstone. Artificial magnets have been armed in a similar manner.
The attractive powers situated in the poles of a natural or artificial magnet seem to be diffused over the two surfaces in which those poles are situated; and the plates of iron which cover those surfaces, serving to condense the powers, cause them to act with united energies in the two feet of the apparatus, by which means the power is greatly in creased. The feet of the side plates should not however be made to approach each other too nearly ; since in such position they would diminish each other's attractive powers.
It is found that a magnet when armed exerts a force more than one hundred times as great as that of the magnet in its natural state ; and it has been ascertained that, after as much weight has been applied as the armed magnet will suspend, if at certain intervals of time other weights be added, the magnet will suspend two or three times as much as before.
Daniel Bernoulli discovered by experiment that the attractive forces of magnets vary in proportion to their superficies and not to their masses ; a circumstance in accordance with Mr. Barlow's discovery respecting the magnetic attractions of iron spheres.