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

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.