I made a helmholtz coil of wire 18 gauge, 420 turns each, 2 cm radius. I passed current of 2-5A and measure the magnetic field with hall probe. When I measured the output, it showed around 100 gauss at 5 A but theoretically, it should be ~900 gauss. Since resistance is small, what could be the reason for such tremendous difference in magnetic strength ? How to maximize the strength?