Re: How does a deflection magnetometer work ?

I guess you are referring to the classical variometer of declination, a 
device that can measure the horizontal component of the geomagnetic field. 
The description of such an aparatus is too complex and full of details in 
order to make an operational instrument, but I'll give you some 
fundamentals of the method, and a reference where you can find an 
extensive description of the classical magnetometers.

The classical variometers are based on the fact that a magnet of moment M 
in a field H experiences a torque L such that

L = M x H (in vectorial notation)

So, a freely suspended magnet with the axis of suspension parallel to the 
z axis (the vertical) can rotate in the xy plane (horizontal) and the 
equilibrium position can be reached when a restoring torque is applied to 
balance the torque L.

You can find information about these variometers in

Laursen, V. & Olsend, J., 1971, Classical methods of geomagnetic 
observatories. In Encyclopaedia of Physics, vol. 49/3, Geophysics III. 
Springer-Verlag, Berlin, Heidelberg, New York.

or

Jacobs, J.A. (Editor), 1987, Geomagnetism, Vol. 1, Academic Press Limited, 
627pp.

Best regards

Eder C. Molina
Dept. of Geophysics
Institute of Astronomy and Geophysics
University of Sao Paulo - BRASIL
eder@iag.usp.br