Let us consider the magnetic field due to current flowing in anticlockwise direction in a circular arc of radius R and length L as shown in figure 1.
Magnetic field B at centre O is given by Biot-savarts law , B =
Hence magnetic field is proportional to arc Length and Current.
If current flows in anti-clockwise direction magnetic field is outward.
If current flows in clockwise direction magnetic field is inward.
In Figure.2, if anti-clockwise direction current flowing in minor arc produces 0.1T magnetic field at O in outward direction,
Magnitude of Magnetic field at O due to clockwise direction current flowing in major arc depends on the product of current and arc length.
Since arc length of major arc is 5 times of arc length of minor arc, its resistance increases by 5 times , hence current in major arc becomes (1/5) of current flowing in minor arc.
Hence the product of ( current × arc Length ) remains same, because current becomes 1/5 and length becomes 5 times.
Hence magnitude of magnetic field will be same.
But due to change in current direction, direction of magnetic field due to major arc is inward.