Helmholtz coils are useful for creating near-uniform magnetic fields or for cancelling out external magnetic fields, such as the earth's. The field between the two solenoids is at its most uniform when the distance between them is equal to the radius of the solenoids. This can also be used with a helium-filled vacuum tube that has been introduced to electrons (not included). When placed between the solenoids into the magnetic field, you will be able to observe the circular movement of the field.