Saturday, September 14, 2013

Determine a magnet polarity

Knowing the polarity of a magnet is important in order to use the same polarity when more than one magnet are used in a motor for example.
Here some notes about how to determine the polarity of a magnet.

There are many different ways to determine the polarity of a magnet :

  • using a compass or equivalent
  • suspending the magnet freely and looking at the final direction (it is assumed to know where the Earth North pole is)
  • comparing it with a known magnet
The most important thing is to know something about the standard.
In USA the "North" pole of a magnet, defined by the National Bureau of Standards (NBS) convention,is based on the following: 

"The North Pole of a magnet is that pole which is attracted to the geographic North Pole. 
Therefore, the North Pole of a magnet will repel the north seeking pole of a magnetic compass."

The magnets

For the experiment we'll be using round shape magnets, easier to be glued to a rotor.
Something like the ones in the picture.

On a disc, the poles are present on the surface.

Marking the poles

In order to figure out what pole is on a surface, the fastest method is to use a compass or an equivalent electronic one.
Pointing the disc surface toward the compass, will force one of the two side of the row toward the disc.
The North pole of the magnet will attract the South pole of the compass.
The South pole of the magnet will attract the North pole of the compass.

Since all the smartphones have a magnetic sensor (Hall effect sensor) it is relatively easy to build an application capable to identify a pole.
On the market there are already many different applications that do so.
For the purpose to identify what is the pole of a magnet we choose the Magnetic Pole Detector.
With this application is easy to identify the pole of a magnet.

It is enough to place the magnet with one surface toward the smartphone and push a button to see what is the pole.

The pictures show a measurement.
The face of the magnet oriented toward the phone, is the South pole of the magnet.
The opposite side is the North pole.

There are of course also many specific tools for this purpose if you down own or want to use a smartphone.

Here few examples :

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