# Measuring Earth's magnetic field

• hasan_researc
In summary: The wire is deflected because there is a force on it that is perpendicular to the current and the wire path is a circular arc with maximum displacement at the center. Because the wire is deflected, it is more accurate to measure the deflection at the center. The power supply that Bob needs to control the current is needed to stabilize the voltage or the current. If the wire resistance varies with wire temperature, a pulley needs to be at only one end of the wire.
hasan_researc
Am reading the attached labscript. Finding it difficult to understand at places. Any good sources of further explanation on the Net or textbooks?

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• ChargesAndFields_LabScript_Sep10.pdf
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could you tell us what your stuck on .

I have to measure the deflection of the centre of the wire. I did so using a microscope but the results are all spread about zero.

Secondly, I have to use a microscope graticule. I don't know why I have to and how to use it.

Your measurement of the Earth's magnetic field uses the same technique as the floating wire method used to map large experimental physics magnets. Look under floating wire magnet in Google.
This a difficult measurement, even in large magnetic fields. Yo need to use a non-magnetic wire with low mass per unit length, low resistance per unit length, and high melting temperature. Copper might be a better choice than tungsten*. Also, a multi-filament wire is better than a solid wire, because it is more flexible. Finally, minimizing pulley bearing friction is very important. Pulley should be electrically insulating if possible. I vaguely recall using a 40Ga. multi-filament copper wire at currents up to ~ 3 amps (it gets very hot). (Maybe tungsten wire is better because you can run it red hot, even though the resistance goes up??).

Finally, measure wire deflection with both polarities of wire current.

* The room-temperature resistivity of tungsten is about 3 x the resistivity of copper.

Bob S

A graticule is a ruled scale on a transparent background that goes inside the microscope so you can measure absolute distances.

Talk to your TA or prof to see if you've set things up correctly. Try taking the mean of your measurements and see if they really are close to zero.

Thanks, everyone.

Now, I'd like to turn to another statement made in section 2 of the labscript:

"In the experiment described here a current carrying wire is held under tension by a small weight (≈ 20 g) and passes over fixed pulleys at either end. It is thus deflected in an arc with the maximum deflection at the centre of the wire. "

I don't understand why the wire is deflected in an arc, and assuming the wire is deflected, why does it have to be max at the centre?

Thanks for all your help!

Secondly,

"You will need a power supply to control the current, and an ammeter and voltmeter to monitor the current through the wire and voltage across it. The power supply can stabilise either the voltage or the current. Wire your circuit and investigate the two modes of operation. Which one is more suitable to do the experiment? "

I am wondering which one would be more suitable?

hasan_researc said:
Secondly,

"You will need a power supply to control the current, and an ammeter and voltmeter to monitor the current through the wire and voltage across it. The power supply can stabilise either the voltage or the current. Wire your circuit and investigate the two modes of operation. Which one is more suitable to do the experiment? "

I am wondering which one would be more suitable?
What causes the deflection?

The force on the wire is due to the Lorentz force law; F = I x B, (vector cross product, so force is perpendicular to both current and field). Because the force is perpendicular to current, the wire path is a circular arc with maximum displacement at center. The displacement at the center is proportional to current, proportional to arc length squared*, and inversely proportional to wire tension.

What kind of power supply do you need if the wire resistance varies with wire temperature?

You need a pulley at only one end.

*Using your knowledge of circles, arcs, and chords you learned in plane geometry, calculate the distance between arc and chord at the center. How does separation depend on the length of chord?

Bob S

## 1. How is Earth's magnetic field measured?

Earth's magnetic field is measured using a device called a magnetometer. This instrument measures the strength and direction of a magnetic field at a specific location.

## 2. What does Earth's magnetic field tell us?

Earth's magnetic field provides important information about the structure and dynamics of our planet's interior. It also plays a crucial role in protecting Earth from harmful solar radiation.

## 3. How is the strength of Earth's magnetic field changing over time?

The strength of Earth's magnetic field has been decreasing over the past few centuries. However, the rate of this change is not consistent and can vary in different parts of the world.

## 4. Can Earth's magnetic field be measured from space?

Yes, Earth's magnetic field can be measured from space using satellites equipped with magnetometers. These satellites provide a more comprehensive and accurate view of the magnetic field compared to ground-based measurements.

## 5. How is Earth's magnetic field used in navigation?

Earth's magnetic field is used as a natural compass and has been used for navigation for centuries. It is also used in modern navigation systems, such as GPS, to determine location and direction.

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