PERFORMANCE ASSIGNMENT

Building a Magnetometer Background

A magnetometer is a scientific instrument used to measure magnetic field strength. On land, magnetometers can be used to find iron ore deposits for mining. Under the sea, marine geophysicists, ocean engineers and nautical archeologists use marine magnetometers to detect variations in the total magnetic field of the underlying seafloor.

Usually, the increased magnetization is caused by the presence of ferrous (unoxidized) iron on the seafloor, whether from a shipwrecked boat made of steel or a volcanic rock containing grains of magnetite, a highly magnetic mineral.

After corrections are made to measurements of the total magnetic field, scientists can use magnetic data to estimate the age and thickness of volcanic lava flows at mid-ocean ridges and ocean island hot spots; to locate pipelines, undersea cables and bridge foundations; and to identify important archeological sites.

Different types of marine magnetometers require different platforms. Surface-towed magnetometers are towed close to the surface of the ocean and far enough behind the ship so that the ship’s magnetization does not interfere with the measurements. Deep-towed magnetometers are often attached to a remotely-operated vehicle (such as the WHOI Towed Camera System).

Surface-towed magnetometers can cover larger areas and are relatively inexpensive. Deep-towed magnetometers can cover detailed areas of the seafloor and have higher sensitivities than surface-towed magnetometers.

Procedure

1. Clean the jar thoroughly and remove labeling.

2. Pierce a small hole in the center of the cap.

3. Glue the 1" magnet to the center of the top edge of the card (1" x 0.5").

4. Label the ends of the magnet by marking one end of the card "A" and the other "B".

5. Glue the piece of soda straw (1" in length) to the top of the magnet.

6. Thread the thread through the soda straw and tie it into a small triangle with 2 inch sides.

7. Tie remaining thread to the top of the triangle in #7 and thread it through the hole in the cap.

8. Put the jar top and bottom together so that the 'sensor card' is free to swing in response to the magnetic fields.

9. Glue the thread through the cap in place.

Practice using the Magnetometer and answer the following questions:

What is the magnet in the bottle sensing?



How long must you wait after placing magnetometer to get a reliable orientation observation?



How does rotating the bottle while making an observation affect the reliability of the measurement?



How does your presence in the vicinity affect the reliability of the measurement?



Can non-magnetic materials cause deflections?



How far must one magnetometer be from another in order to keep them from interfering?