Fields in Helmholz coils

Courtesy of Chuck Reese

The purpose of this demonstration is to show the direction of the magnetic field lines within the Helmholz coils, and to impress upon the students their behavior as electromagnets. With these particular Helmholz coils, a current of 5 Amperes provides a strong enough field to perform the demonstration. At this current, the coils get warm if left for more than about 10 minutes, so it is best not to use larger currents. As shown above, the coils are connected in parallel to the power supply, so the total current provided by the power supply is 10 A.

A Helmholz coil is a short, fat (large-diameter) solenoid. As in any solenoid, the magnetic field lines loop around the coil, and within the coil and next to it along its length, the field lines are parallel to the axis of the coil. Thus, when the coils have current flowing in them, they behave as bar magnets, with a north pole on one side and a south pole on the opposite side. You can illustrate this by moving a bar magnet on a swivel all around the coil, inside and outside, or the magnetic dip needle. Removing the base from the magnet on the swivel allows greater freedom in moving it within the coil. When you place the magnet near the open area of the coil, it will align itself parallel to the coil axis, according to whether it is on the north or south side of the coil.

When the coils are connected as shown and a 5-A current applied to each, when the coils are facing the same direction, i.e., the north pole of one faces the south pole of the other, the two coils attract each other, and you can pull the hanging coil to one side, perhaps to an angle of about 30 degrees, by bringing the freestanding coil near it. When the two coils point in opposite directions, so that their north poles or south poles face each other, the coils repel. By bringing the freestanding coil next to the hanging coil, you can push the hanging coil aside, to an angle of perhaps as great as 30 degrees, with a gap of about 1/2 inch between the coils. The gap is visible to the students in the lecture hall, so they can see that the coils are actually repelling each other, and that you are not merely supporting one with the other.