Interesting picture depicts Fig. 1, reproduced from a photograph from his hands, put on the poles of an electromagnet, sticking up bundles "of large nails, like hair hard. Itself completely hand does not feel the magnetic force: invisible threads pass through it, does not betray its presence. And iron nails obediently obey its impact and are arranged in a certain order, revealing to us the direction of the magnetic forces.
Fig. 1. The magnetic forces pass through the hand.
The person does not have a magnetic sense organ; therefore, the existence of magnetic force that surround the magnet, we can only guess *. However, it is easy indirectly discover a picture of the distribution of these forces. This is best done using a small iron filings. Sprinkle sawdust thin even layer on a piece of smooth cardboard or glass plate; move under cardboard or vinyl ordinary magnet and shake the sawdust easy blows. Magnetic force can pass through the cardboard and glass; therefore, iron filings under the action of the magnet are magnetized; when we shake them, they are momentarily separated from the plate, and can under the action of magnetic forces turn easily, taking the position that would take at this point the magnetic needle, i.e., along the magnetic "lines of force". In the sawdust are arranged in rows, clearly detecting the distribution of invisible magnetic lines.
Place over our magnet plate with sawdust and will stir up. We will provide the shape shown in Fig. 2. The magnetic forces create a complex system of curved lines.
Fig. 2. The arrangement of iron filings on the cardboard covering the poles of a magnet.
You see, radiantly as they diverge from each pole of the magnet, as sawdust are interconnected, forming the short and long arc between the two poles. Iron filings personally show here that mentally draws himself a physicist and the invisible present around each magnet. The closer to the pole, the line of sawdust thicker and more distinct; on the contrary, with the distance from the pole they razbegautsia and lose their distinctness, clearly proving the weakening of the magnetic force with distance.
* It is interesting to imagine what was going through would we be if had a direct magnetic sense. Cradle managed, so to speak, to instill cancers genus magnetic sense. He noticed that the young crayfish put his ear small stones; these stones its weight acting on the sensitive hairs, forming part of the organ of equilibrium cancer. Similar stones, called otoliths, and have someone's ear, close to the main organ of hearing. Acting in the vertical direction, these stones indicate the direction of gravity. Instead of the stones Cradle planted cancers iron filings, which they had not noticed. When touching the magnet to cancer last was located in the plane perpendicular to the component of the magnetic force and gravity.
"The last time the corresponding experiments in a modified form was able to produce and over man. Köhler glued a small iron particles to ear the eardrum; this ear perceived fluctuations of the magnetic force as the sound". (Prof. O. Winer.).