EXPERIMENT 50
Magnetic needle
The bar magnet is only somewhat useful
as a compass, since its shape won’t let
you read the direction very precisely. But
there’s a better option. Try your hand at
making a replica of one of the earliest
compass models — a floating compass.
HERE’S HOW
Fill a bowl with water. Break off a piece of
polystyrene foam, as flat as possible,
from the kit’s parts tray, and let it float in
the bowl.
Once it is floating properly, insert the
magnetized needle from Experiment 46
(Birth of a magnet) horizontally through
part of the polystyrene foam piece, and
let it float freely in the middle of the
bowl. What do you notice?
How does your floating compass react
when you bring the ring magnet or bar
magnet close to it?
WHAT’S HAPPENING
If the polystyrene foam floats freely, the nee-
dle always points in a north-south direction.
Slender arrows and needles let you detect a
northerly direction more precisely.
In fact, early compasses really were made
out of a magnetized needle and cork float-
ing in a bowl of water marked with a scale.
48
N
N
S
S
ALWAYS TO THE
NORTH
A compass needle, as
you already learned, is
a tiny magnet. It ori-
ents itself according to
the direction of Earth,
since Earth itself is a
magnet.
Earth behaves as if
there were a giant bar
magnet wrapped in-
side its interior, with
one end near the North
Pole, and the other end
near the South Pole.
In reality, of course, it
isn’t really a gigantic
permanent magnet
that produces Earth’s
magnetic field. The
field is actually
caused by powerful
electrical currents
flowing through
Earth’s metallic core.
N
S
Summary of Contents for Electricity and Magnetism 620417
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