Question

why does the deflection in the galvanometer oppose the direction of the magnetic field?

Answer 1

Explanation:

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Answer 2

Buddy!  A moving coil galvanometer works on the principle that a current-carrying coil placed in a magnetic field, experiences a torque (or force for simplicity). The coil springs along with the field ensure the deflection to be proportional to the strength of the current.    

Now, The direction is always opposite to the magnet due to Lenz's law.  

lenz law formula:   ε= -N (δΦB)/δt        

This is signified in the formula by the negative sign (‘–’).