Comparing the magnetic field of a permanent bar magnet, depicted in figure. 22.8, with the magnetic field of the solenoid, shown in figure. 22.7, you can see, these fields are in appearance the same. The difference between them is only inside the solenoid and magnet (the location of the lines of induction in the magnet could not be seen).
Magnetic properties of the solenoid with a current and a bar magnet is almost the same. For example, if a solenoid be suspended so, so he could rotate in the horizontal plane, he is established in the direction from North to South. It is good according tothat with the idea Ampere, according to which field magnet is created by currents of molecules. All the above gives the right to assert, what line induction field of the magnet are closed, t. e. continue inside as well, as in the solenoid (rice. 22.9).
Since the magnet has poles, we can conclude, they should be at the solenoid. Indeed, if solenoid with current of one end is attracted to the North pole of the magnet, the other end is repelled from it. Selecting the right screw rule, the direction of the lines of induction, you can define the magnetic poles of the solenoid: like magnet, induction line out of the solenoid from North pole and enter from the South.
As determined by the "poles" of the circuit with current (he has a "pole" is the surface, streamlined current). Based on the above we can give the following definition of the pole of the solenoid or circular current: the surface, which the current circulates counterclockwise, is the North pole, and the surface, streamlined current clockwise, is the South pole (rice. 22.10, and, b). The magnetic poles of the coil shown in Fig. 22.10, in. The poles of the solenoid shown in Fig. 22.7.
Note, when on the same circuit looking from two opposite sides, for one observer the current in the loop goes clockwise, and the other counterclockwise. Therefore, each circuit with a current has two different poles. Thus, magnetic poles exist only in pairs.
Any way it is impossible to obtain a single magnetic pole. If a permanent magnet break, you get two magnets with North and South poles of each (rice. 22.11).