### Re: How we could measure magnetic field strength of earth at a particular point

Date: Wed Feb 24 10:58:58 1999
Posted By: John Balbach, Post-doc/Fellow, Physics, National Institutes of Health
Area of science: Physics
ID: 919068654.Ph
Message:
```
Mr. Rehan,
The easiest way to measure the Earth's magnetic field is to buy a
gauss meter and follow the instructions.  The harder way is to build your
own.  There are three ways I can think of to do it.
The first way is to build a rotating coil apparatus.  When you rotate
a coil in a magnetic field, there will be a voltage induced in the coil
that will depend on the angle of the coil makes with the magnetic field.
As the coil rotates, this voltage will oscillate, so you should either
measure the volate with an oscilloscope, or use the "AC" setting on a
voltmeter.  Here's the catch: this is not easy to do.  For a 10-turn coil
that is 1cm across rotating at 50Hz, the peak-to-peak voltage will be
something like 100 microvolts.  That is assuming you are able to build an
apparatus that can rotate a coil at a known frequency while measuring the
voltage across the coil.
The physics behind this is known as Lenz's Law.  The law states that
the induced voltage is proportional to the time rate of change of the
magnetic flux.  Translated, that means if you choose the right units (mks)
the peak-to-peak voltage in a rotating coil will be equal to the product of
the magnetic field strength, the area of the coil, and the rotational
frequency of the coil (in radians per second).
The second way would be to find the nuclear magnetic resonance
frequency of the hydrogen nuclei in a bucket of water (~2 kHz in the
earth's field).  To do this would take a lot of knowledge about building
antennas, detecting radio signals and the like.  This is also a difficult
process.
The third way would be to measure the torque on a magnetic dipole
(like a compass needle or other weak magnet).  This suffers from the
difficulty of calibration.  Forget I mentioned it.
So the answer to your first question is: with great difficulty and
expense (in time and/or money).  The answer to your second question is:
move the objects away.  I can think of no quantitative way to allow for the
effects of nearby ferrous objects.  The upside of this is that for any
application where it is important to know the precise magnetic field at a
given point, you will need to include the effects of nearby ferrous
objects.
If you decide to buy a gauss meter, the earth's field is roughly 0.5
Gauss.  Keep this in mind when shopping for the appropriate instrument.

```

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