Re: How does a null detector work in an electronic balance?

In an electronic balance, the pan has much more limited travel than in a 
mechanical balance.  Upon placing a load (weight) on the pan, there will 
be a small amount of displacement.  This is measured, probably with a 
light source (small laser, perhaps?)  A current is applied to a coil under 
the pan, which moves the pan up and down (right hand rule gives you the 
direction of the force).  By determining the amount of current it takes to 
hold the pan at its original, undeflected position, one can calculate the 
electronic force necessary which is equal to the mechanical force exerted 
by the load.  The null detector is most probably the part of the circuitry 
responsible for maintaining the constant pan position, and would encompass 
the light source, light detector (probably a photodiode), and circuit 
which makes voltage comparisons and relays the need for more or less 
current to the coil.

I hope this helps
Please feel free to email me if you have further questions: 
weibelm@battelle.org

Best Regards,
Mike