# How does magnetic field vary with distance?

This Instructabler is a scientific investigation to determine how magnetic field varies with distance. Two methods are presented , and reasonable conclusions made.

The magnet is a thin disk type, not a bar magnet.

1. Tools and equipment:
2. One rare earth neodymium magnet, 16mm diameter x 3mm.
3. Precise scale measuring to 0.1 gram
4. Balance beam apparatus to allow measuring magnetic attraction
5. Magnetic compass
6. Tape measure (non magnetic)

Note: This is a revision of an earlier Instructable by the same author. New data, analysis, and conclusions are presented to update and correct the previous Instructable, Determine how magnetic field varies with distance.

Some changes were also made in order to qualify for the Scientific Method contest on Instructables.com.

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## Step 1: Background: inverse square law?

Many phenomena of nature, like light, obey the inverse square law. That means as you get farther away from the source of light, the intensity decreases as the square of the distance. The inverse square lawapplies to light, gravity, and electrostatic charge. And the equation is simpleand beautiful: basically it is I = 1/d2, where d is distance.

It is often assumed that the strength of a magnetic field also obeys the inverse square law.
Researching the Internet produces many complex equations, some suggesting that magnet field varies inversely with the third power of distance, in other words an inverse cube law.

I decided to test for myself.

rimar20007 months ago

Brilliant experiment, Bill.

I don't understand how you measure the magnetic force using a compass.

Bill WW (author)  rimar20007 months ago

Gracias, señor Osvaldo

I should have mentioned the source of the method I used for measuring magnetic field with a compass. Here it is: