MadSci Network: Earth Sciences

Re: Compass Experiment - How can you magnetize a needl

Date: Wed May 9 12:52:41 2001
Posted By: David Smith, Faculty Geology, Environmental Science
Area of science: Earth Sciences
ID: 988561135.Es

This one had me stumped for a while.  Aside from happening to find 
magnetite ore (the old fashioned "lodestone" that mariners used), I wasn't 
sure how you could do this.  If you happen to be backpacking near an old 
magnetite mine, you'd be all set, but magnetite ore isn't exactly common 
(magnetite is, but there is usually not enough of it in a rock to give 
significant magnetism).  I did some poking around and I found one curious 
reference from Moby Dick in which tapping an iron shaft causes it to become 
magnetized and then it can be used to fashion a compass.  See:

Impacts can damage magnets and diminish their strength, but I have never 
heard of it being used to induce magnetism.  As I said, curious.

You really need a magnetic field of some sort to magnetize your needle.  
One way to generate a magnetic field is with an electric current.  A 
straight current-carrying wire has a circular field induced around it.  If 
you coil the wire, you produce a linear field along the axis of the coil 
(this device is known as a solenoid).  An iron or steel core in the 
solenoid produces a classic electromagnet.  So, if you had an bit of 
insulated wire and bolt or one of the pins that sometimes hold packs to 
frames (as long as it isn't aluminum) and a battery, you should be 
able to fashion an electromagnet and use that to magnetize your needle.  I 
have to say that I have never tried to do this, but the American Academy on 
Physics Teaching publishes a book of activities (Teaching About Magnetism, 
by Robert Reiland) which includes using a solenoid to magnetize a needle 
and then map out the magnetic field:

This should have details on how to do it.

David Smith
Department of Geology, Environmental Science, and Physics
La Salle University, Philadelphia, PA

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