The Coil

The figure below shows the shape of the magnetic field around the wire. In this figure, imagine that you have cut the wire and are looking at it end-on. The green circle in the figure is the cross-section of the wire itself. A circular magnetic field develops around the wire, as shown by the circular lines in the illustration below. The field weakens as you move away from the wire (so the lines are farther apart as they get farther from the wire). You can see that the field is perpendicular to the wire and that the field's direction depends on which direction the current is flowing in the wire. The compass needle aligns itself with this field (perpendicular to the wire). Using the contraption you created in the previous section, if you flip the battery around and repeat the experiment, you will see that the compass needle aligns itself in the opposite direction.

Magnetic field of a wire

Because the magnetic field around a wire is circular and perpendicular to the wire, an easy way to amplify the wire's magnetic field is to coil the wire, as shown below:

One loop's magnetic field

For example, if you wrap your wire around a nail 10 times, connect the wire to the battery and bring one end of the nail near the compass, you will find that it has a much larger effect on the compass. In fact, the nail behaves just like a bar magnet.

A simple electromagnet

However, the magnet exists only when the current is flowing from the battery. What you have created is an electromagnet! You will find that this magnet is able to pick up small steel things like paper clips, staples and thumb tacks.