To measure the Earth's magnetic field, special instruments must be used. There are two types of such instruments.

First, there are mechanical instruments like the compass. The compass can tell us where magnetic north is at any given time and place. This can help us measure the declination of the magnetic field.

Second, there are non-mechanical instruments like magnetometers. These can measure things such as the strength of the magnetic field.

By using both of these kinds of instruments, we can find the direction and the strength of the Earth's magnetic field.

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**Measurement of Earth’s Magnetic Field **

Purpose:

**To measure the horizontal component of the earth’s magnetic field. **

Equipment:

**Tangent galvanometer, 6 volts power supply, milliammeter, reversing switch, connecting wires. **

Introduction:

**As Oersted showed in the 19th century, a magnetic field is produced whenever a current flows through a wire. The magnitude and direction of the field at points near the wire depends on the shape of the wire as well as the amount of current flowing through the wire. One particularly useful geometry that commonly occurs is a single circular loop of wire. The magnetic field, B, at the center of such a loop is given by **

Where μo = 4

π x 10-7 Tesla-m/amp, I is the current in amps and R is the radius of the loop in meters. The direction of B is given by the right hand rule.

On the surface of the earth, the magnetic field at the center of such a current loop is the resultant of two fields: that due to the current in the loop and that due to the magnetic field from the earth as shown in figure 1. By determining the direction of the resultant magnetic field we can, using equation 2 and vector addition, determine the horizontal component of the earth’s magnetic field.

The direction of the magnetic field can be determined at any point by observing the direction of a compass needle at that point. Therefore by placing a compass needle at the center of a circular wire loop we can determine the direction of the magnetic field at that location. With no current flowing the compass will be aligned with the direction of the horizontal component of the earth’s magnetic field, Be.

When the current is turned on, the compass needle will deflect through an angle

θ

to realize itself with the new resultant field. The angle of

deflection will of course depend on the size of the magnetic field Bc caused by the current in the wire loop. By measuring

θ

one can calculate Be from the equation (see figure 1 ).

Be = Bc/Tan

θ ………………….. ( 2 )

6 v

Figure 2

Procedure:

1. Choose a location in the room as far as possible from any magnetic materials (iron, pipes, etc). Align the plane of the current coil of the tangent galvanometer as closely as possible with direction of the earth’s magnetic field as shown by the compass at the center of the coil.

2. Set up the circuit shown in figure 2 and carefully increases the voltage on the power supply until the current through the coil causes a deflection of approximately 35o. Record the value of the current. Read the deflection angle,

θ1, as closely as possible, then throw the reversing switch to reverse the direction of the current and carefully measure the new angle, θ2. By measuring the deflection for both current directions the error introduced by inexact alignment of the coil with the earth’s magnetic field is reduced.

3. Using equations 1 and 2, find the average Be (from forward and reverse currents) for approximate deflection angles of 35o, 40o, 45o, 50o, and 55o. Find the percent difference from the mean for each of the five values obtained for Be. If possible, obtain a local value for Be and compare with your results.

Measurements of the magnetic field of the Earth are made by the magnetic observatories. To these observatories, located on the surface of the Earth, we'll have to add the measurements made by the specialized artificial satellites. These satellites, which oversees the Earth's magnetic field, moment by moment, in the last nine years, have achieved spectacular results about the evolution of Earth's magnetic field.

Gathering all the measurements performed so far, we have an extraordinarily large number of data, but also a very good quality of the recordings of Earth's magnetic field.