How magnetic is it? Measuring magnetic pull with a compass
The question
Can the distance at which a sample deflects a compass needle rank rocks and metals by how much magnetic material they contain?
Iron meteorites and most stony meteorites contain iron–nickel metal and attract a magnet, while most Earth rocks do not. A few Earth rocks rich in magnetite are strongly magnetic too, which is why a magnet alone cannot prove a find is a meteorite.
Equipment
- A magnetic compass on a sheet of paper, well away from metal tables, phones and steel
- A ruler fixed to the paper pointing east–west at the compass
- Samples: a steel bolt, iron filings sealed in a small bag, basalt, granite, sandstone, a magnetite pebble or lodestone if available, and any slag
- A balance
Method
- Let the compass settle pointing north.
- Slide each sample slowly along the ruler towards the compass from 30 cm away.
- Record the distance at which the needle first moves by 5 degrees.
- Repeat three times, then weigh each sample so you can compare samples of different sizes fairly.
- What to measure
- The deflection distance in centimetres (a larger distance means a stronger effect), and the mass of each sample.
- How to present it
- A ranked bar chart of deflection distances, and a discussion of why magnetite-rich Earth rocks and slag can fool meteorite hunters.
- Take it further
- Try equal masses of each material by weighing out small amounts, and see whether the ranking changes. Read the meteor-wrongs gallery and find which look-alikes are magnetic.
Safety
Keep magnets and magnetic samples away from phones, bank cards and anyone with a pacemaker. Keep iron filings sealed in their bag.