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Magnetic Compass
Name: Bob
Status: other
Age: 30s
Location: N/A
Country: N/A
Date: 2000-2001
Question:
I am designing a crude compass. I have used an array of
16 ceramic permanent magnets cylindrical in design 3/8" long by 3/4" in
diameter stacked together for a total of 6" in length. I have affixed a
3/16" threaded brass balancing road in line with this with nuts to balance.
I have suspended this large compass needle from an alumininum strap "c"
frame by fishing line and tackle swivels. This system seems to generally
work as the needle seems to point towards the magnetic north pole ( I am
located within an equipment/office building ). The problem is with there
is no dampening affect with this fish tackle suspension system and my
homemade compass needle seems to oscillate between East and West for hours
before settling down on magnetic North. I thought by using such powerful
magnets the direction finding power would be quick and powerful. Not the
case I am afraid as the reverse has become the case.
The compass takes hours to settle down. Do stacked ceramic cylindrical
magnets make good compass needles ? I have heard that while they obviously
have a N&S pole, the manufacturing process leaves this polarity out of
true by upto many degrees leaving an polarity angular and offset to the
cylindrical design. Is it feasible to use ceramic magnets? Does the fact
that these magnets are not actually created/molded as on contiguous unit
as a opposed to being attracted by stacking in a N/S
attraction/orientation have any bearing? Would a steel conduit sheath
around the needle assembly have a polarity focusing affect to enhance the
orienting affect as seen in say cupboard latchs?
Replies:
Your problem is not that there is something wrong with your magnets, but
that your bearing is too good. What you are observing is harmonic motion.
When your compass needle is out of alignment with the earth's magnetic
field, it experiences a torque pulling it toward the anti-parallel
orientation. As it approaches this optimum anti-parallel orientation, the
torque gets smaller and smaller.
However, as it moves toward the optimum orientation, it is picking up speed!
It turns out that the needle is moving the fastest when there in no force
acting on it, that is, when the needle is in the optimum anti-parallel
position. The momentum of the moving needle keeps it swinging right on
through this optimum position, and again the earth's magnetic field will
pull it toward the optimum orientation, this time slowing its motion. The
needle then decelerates and the restoring torque gets larger as the needle
gets further and further away from its optimum orientation. And so on,
until friction from your bearing finally dissipates all the system's kinetic
energy.
Put another way, when the needle isn't pointing due north/south, it has a
higher potential energy than when it is. As it swings toward the optimum
orientation, it converts potential energy into kinetic energy. When the
needle points due north/south, its potential energy is lowest. Total energy
(kinetic + potential) is conserved, however, so its kinetic energy is
highest here, and is equal to the potential energy difference between the
extreme and optimum positions.
What you need is a mechanism that will impose friction to your moving
needle, so that it eventually stops swinging. You don't want significant
friction on the needle when it is moving slowly, because then it may stop
before it reaches the optimum orientation. Common engineering solutions to
this problem include liquid or magnetic dampers.
Richard E. Barrans Jr., Ph.D.
Assistant Director
PG Research Foundation, Darien, Illinois
While I can't answer most of your questions, your LARGE compass has a large
moment of inertia, so that once it starts swinging on a low friction pivot
point it will take some time for friction to overcome the oscillations. You
need to think of some way of dampening the oscillations to shorten the
equilibrium time.
Vince Calder
Sounds like you have designed a nearly frictionless suspension for the needle.
A perfectly frictionless system would not lose energy and the oscillations would
continue forever. The more friction the quicker it will settle. Of course,
with too much friction it will not be able to move into the magnetic north
direction.
Many manufactured compasses are in oil or some other viscous liquid. The
energy
is lost to the liquid but the friction is low enough to allow the compass
needle to swing toward magnetic north.
Greg Bradburn
For feeling the magnetic force, your magnet is fine. If there were no air
around it, your magnet would oscillate forever, as would any other compass
needle. This is precisely why compasses tend to have very light needles.
The air resistance slows them down quickly. You need more air resistance.
Try hanging a piece of notebook paper from the needle or the C-frame. The
resistance caused by the paper pushing air out of the way will slow the
oscillations, but the tiny mass of the paper will not add any extra inertia
to the system.
Kenneth Mellendorf
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