Horseshoe magnets originated from the idea of bending a magnetized rod, thus bringing the two poles close together so that they could act simultaneously on the same armature and double the load they could lift; it was found, however, that a horseshoe magnet could lift more than twice the load that one of its poles could lift by itself.
The explanation of this lies in the fact that when the armature is placed in contact with only one of the poles of a horseshoe magnet the polarization of the of the extremity not in contact is not exploited. But when the armature is in contact with both poles, since polarization is the overlapping of the polarizations that it would undergo if it were placed in contact with the two poles separately, we have the exploitation of the polarization that each pole produces at the extremity of the armature with which it is not in contact; as a consequence the force of attraction that each of the poles of the magnet exerts on the armature is greater than the force it would exert if it acted alone. The example presented here is made of tempered steel, equipped with armature and is quite wide (width 28 cm; length 21.5 cm).
The arms are 3 cm wide and 1 cm thick. The extremities of the armature not in contact are worked and it has a robust hook.
Daguin (1878), T. III, p. 47
Drion-Fernet (1883), p. 440
Ganot (1864), p. 500