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Helmholtz Coils

We can obtain a close approximation to a uniform magnetic field with pair of circular coils on the same axis with equal currents flowing in the same sense. The coil separation needed to give the most uniform central field is equal to the coil radius. The magnetic field at the centre of the coils is  
\[B=\mu_0 \frac{8IN}{5 \sqrt{5} R}\]

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