What is snow rule or for what purpose this rule is used

 

Snow rule states that if the current is flowing in an electric circuit from South to North direction

and magnetic compass is placed Over the conducting wire, the needle of the compass deflects in the West direction

 

The name of the rule is chosen so that to remeber the words :- S for South, N for north, O for over and W for West .

These letters are typed in bold letters in above statement.

 

Purpose of the rule is to find the direction of magnetic field produced by the current carrying conductor.

 

When current is flowing in the direction of south to north, magnetic field lines are circling around the conductor and the magnetic field directionabove the coductor  in the circular line is from east to west. Hence the field lines are such that as if a magnetic north pole is placed in the east. Hence the compass needle placed above the conductor shift towrads west.