One can not align the apparatus to eliminate entirely the effects of the Earth\'

Question

One can not align the apparatus to eliminate entirely the effects of the Earth's magnetic field. Consider the case where electrons having accelerated through V then enter the uniform field region between the Helmholtz coils. With the coil current set to zero, the electron beam was observed to have a slight curvature due to the earth's magnetic field. The current in the coil had to be set to 0.485 A to set the electron beam exactly straight. If each coil has 62 turns and has a mean radius of 66.0 cm, calculate the component of the earth magnetic field perpendicular to the entering electron beam:

Im using the equation: B = I *sqrt( (8N[mu]) / (5R sqrt(5))  )

My answer is B = 4.457 mT how ever this is wrong, can you explain the method to finding the solution

Explanation / Answer

permeability of free space = 1.26 * 10-6 T m/A

B = (4/5)^(3/2 ) * [mu * n * I / R]

= 0.7155 * 1.26 * 10-6 * 62 * 0.485 / (66 * 10-2)

= 4.10 * 10-5 T

= 0.041 mT

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