You must have heard the phrase “opposite poles are attracted”. Although this is far from always true for relationships between people, this rule is invariably true for magnets. We are all used to dealing with a large magnet - the Earth. Experiments with small magnets will help you understand how the Earth's magnetic field acts, which protects us from cosmic radiation. Whether you want to conveniently designate the poles of a magnet or conduct an interesting physical experiment, you will need the ability to determine the polarity of magnets.
Permanent Magnet Polarity Indicator
In amateur radio practice, as well as in the design of devices that use magnets and inductors, sometimes it is necessary to find out the polarity of the magnets and the direction of the magnetic fields. The proposed device allows you to determine the polarity of the magnets.
I put together two different options. The main unit of the first device is two rectangular magnets, which are poured into a cylinder, in the center of which a rotation axis is mounted on the bearing. Filling is carried out with epoxy resin. When such an assembly is introduced into the magnetic field of the magnet under study, the assembly rotates with the opposite pole and tries to attract itself to it. A hole was made in the housing in order to visually observe the position of the magnet. The poles of the cylinder assembly are painted in different colors, corresponding to the opposite poles of the magnetic assembly. That is, inside the cylinder, for example, a magnet is located with the north pole to the edge, and on the surface of the cylinder there is a marker of the south pole. Then it will be more convenient for the observer to determine the pole of the magnet under study. The magnetic assembly device is shown in the figure below:
Fig. 1. The internal structure of the magnetic assembly.
To determine the polarity of the magnet - you should bring the device with the disk to the magnet under study. Suppose we brought to the north pole, in this case, the disk will turn to the magnet with the south pole of the first magnet inside the assembly. And in the viewing window, the observer will see the assembly surface painted in blue, which corresponds to the blue pole of the magnet under study. On the contrary, if we bring the device to the south pole of the magnet under study, the assembly will turn to it with the north end of the second rectangular magnet, and the part with the surface painted in red will be opposite the viewing hole.
The second device is more advanced because it has a light indication. A cylindrical magnet is mounted in a rod of non-magnetic material. A ring is installed at its end, to which a positive wire from the battery is attached by soldering. The movement of the magnet inside the rod is limited by the walls on which the contacts for the north and south poles are fixed, as shown in the figure:
Fig. 2 Electronic-mechanical indicator of field polarity. The magnet is stabilized in space by a spring. Two LEDs are mounted in the end part of the housing, indicating the poles of the magnets. Suppose the rod is close to the north pole of the magnet under study. In this case, the magnet inside the rod will tend to be attracted to the magnet under study. In this case, the contact of the magnet rod touches the contacts of the North Pole indicator and closes the power supply circuit of the blue LED. If the magnet under investigation is facing the rod by the south pole, the magnet will be drawn into the rod and press the contact of the magnet rod to the contacts of the south pole and light the LED of the south pole. Also, in order to understand the polarity of the magnets, you can use a conventional compass. And if you make a fully electric indicator, then you can use the circuit on the Hall sensor. The simplest scheme is presented in the figure below (taken from the site http://www.valtar.ru/):
Fig3. Determination of a magnet field based on a Hall sensor.
When there is no magnet - the reed switch is open, the supply voltage to the circuit is not supplied. We bring the magnet to the Hall sensor and the reed switch. The reed switch closes, and, depending on the polarity of the magnet, 0 or the supply voltage appears at the output of the Hall sensor.