# Determining the magnetic field of a bar magnet experimentally?

## Main Question or Discussion Point

I would I go about determining the magnetic field of a bar magnet experimentally? Any ideas?

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Use a many-turn (N > 100) air coil slightly larger than the bar magnet and slip it over the bar magnet down to the middle. Turn on a voltage integrator - see my post # 27 in

and pull the coil off the end and away from the magnet. This measures the volt-seconds of the coil voltage output, which is equal to the magnetic flux Φ. This will measure the total flux Φ out of one end. Divide the measured flux Φ by the area A of the end of the bar magnet to get the flux density B.

Bob S

Awesome thanks a lot!!

Awesome thanks a lot!!
You can double the voltage integrator output signal if you flip the coil over and slide it back on the bar magnet.

Bob S

I am currently in undergraduate Physics, and have little experiance setting up circuits and I feel that is beyond by capabilities due to the lack of knowledge I currently have. Is there any easier way? It doesn't have to be extremely precise.

EDIT:

What if I create an EMF by rotating the magnet in a solenoid and using faradays Law?

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What if I create an EMF by rotating the magnet in a solenoid and using faradays Law?
If the bar magnet is less than ~2 times longer than it is wide, you can do it this way. The equipment for this is very similar to the voltage integrator/ flip coil method I describe above and in the referenced post. The flip coil can be built using parts at the local electronics store (voltmeter, dc power supply, soldering iron, op-amp, resistor, capacitor, wire, coil form).

Here is a list of various methods suitable for your magnetic field measurement application.

Hall Effect
NMR (nuclear magnetic resonance)
Flux gate
Rotating coil
SQUID
Flip coil

Look them up on the web. Here is another method.

Calculate the moment of inertia of your bar magnet.

Tie a thread to it and measure the oscillation period (torsional pendulum) in Earth's magnetic field.

Calculate magnetic moment of bar magnet. From dimension of magnet, calculate the pole tip field strength.

Bob S

Thanks A lot man, I can tell you know your stuff!!