Helmholtz coils are a pair of coils, each having radius R and n turns, and carrying a current I, placed a distance R apart, with their poles aligned. 

The reason for this precise construction is it allows the magnetic field at the on axis point directly between the coils to be computed according to

.

Suppose n=50 and R=0.05m.

What current do you need to generate a field equal to that of the Earth's magnetic field (~40 microteslas)