Louisiana History
1650 - 1700
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Searching the Belle

Activity Sheet

Name ____________________

Magnetic Survey
by James Zuhn

How did archeologists find the Belle under 12 feet of muddy water? Careful measurements of the magnetic field at the surface of the entire bay revealed an anomaly which turned out to be a cannon on board the ship. You can learn how this was done and have a lot of fun at the same time!

You will need the following materials:

  • One sheet of poster board
  • One iron disk (described below)
  • Tape (transparent or masking)
  • Magnetic compass
  • Pencil
  • Ruler

In this activity, you will work in teams of two or three students, challenging other teams to find your "treasure." The treasure will be represented by an iron disk such as those used by carpenters to nail tarpaper (you can be sure if the disk contains iron if it is attracted by a magnet). The water will be represented by the poster board. Tape the disk to the underside of the poster board at least two inches from any edge of the board.

Place the poster board on the table with the disk facing down. Carefully mark dots on the poster board every two inches along either the northern or southern edge of the board.

With that, you are ready let the other team find your "sunken treasure!" To find the treasure, position the compass close to one of the premarked dots with one end of the needle pointing directly to the dot.

Mark where the opposite end of the needle points. Then move the compass and mark another dot as shown below. Use the ruler to draw a line connecting points in the order they are drawn.

Continue this process until your line goes off another edge of the poster board. If your line has not passed over the treasure, the line should be straight, running north to south. When the compass is near the disk, however, the needle will deflect from the straight line. This is because the disk has its own magnetic field, which disturbs the earth's magnetic field nearby. (Before having the second team look for the disk, it is wise to pass the compass over the location of the disk to be sure a deflection can be observed.)

When lines have been constructed starting from all the points along the edge of the poster board, the location of the treasure will be quite apparent.

The instruments used by the archeologists to find the Belle were, of course, much more sensitive than your compass. But this activity is based on the same principle.

 


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