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Activity 3

Measure The Earth's Magnetic Field

We can think of the Earth as a giant magnet with magnetic field lines pointing away from the magnetic north pole and towards the magnetic south pole.


Suppose you are one of the stick people in the figure above. The Earth’s magnetic field line through you is the magnetic field at your position.

Notice the magnetic field lines form an angle with the Earth’s surface. We can visualize this angle by utilizing the magnetometer on our phone. 

Launch a compass app on your phone and find the magnetic north. (You may need to download an app for this)

We have now aligned our phone magnetometer to the magnetic field line, facing to the magnetic north as shown. 


In the figure, the compass points towards the magnetic north. The red arrow represents the total magnetic field. The total magnetic field points towards the magnetic north and slightly downwards in the Northern hemisphere and slightly upwards in the Southern hemisphere. This angle between the total magnetic field and the Earth’s surface is called the inclination angle. The inclination angles change as we move along the hemisphere!


This example illustrates the position at the Northern hemisphere.
How would it be like at the Southern hemisphere and along the equator?

  1. Still facing the magnetic north, hold your phone horizontally with the screen facing up.

  2. Launch the Phyphox magnetometer and select "MULTI". Hit start.

  3. Slowly rotate your phone from left to right (with the screen still facing up) until the x-component is close to zero.

  4. Pause the measurement and record the values. 


From this figure, can you see why the x-component of the magnetic field is zero?

The total magnetic north points slightly downwards and towards the magnetic north. The downwards direction corresponds to the negative z-direction of the magnetometer on our phone and the magnetic north direction corresponds to the y-component of the magnetometer on our phone.

To calculate the total Earth’s magnetic field, use the formula:


What do you notice about your calculated Earth’s magnetic field and the absolute value of the magnetic field on your phone’s magnetometer?

The absolute Earth’s magnetic field should be in the range of 25 μT and 65 μT. 

To calculate the inclination angle, use the formula:


Next, still facing the magnetic north, slowly rotate the phone upwards with the screen facing to yourself until both the x-component and the z-component are zero. Pause the measurement and record the values.


From this figure, can you see why the x- and z-component of the magnetic field are zero?

This is because we now aligned the absolute magnetic field to the y-component of the magnetometer. There is no magnetic field in the x-component and z-component.

Compare the approximate angle between your phone and the ground with the inclination angle that you had calculated.


Upload the measured absolute magnetic field and your calculated inclination angle to us - this will contribute to the map of measured Earth's magnetic field!


Note: The inclination angle decreases to zero as we approach the equator and increases to 90 degrees as we approach the geographical north and south poles.


Note: The magnetic field decreases as we approach the equator and increases as we approach the geographical north and south poles.

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