Fig. 2—Compass Needle Test
It is not necessary that the magnetic field be created by a permanent magnet. It can be produced by a current in a conductor. The fact that there is a magnetic field surrounding a conductor in which there is a current can be shown by a simple experiment, as illustrated in Fig. 2. If a wire be placed above a compass needle and parallel to the direction of the compass needle and a current be sent through the wire in the direction indicated by the arrow I, there will be a force acting on the compass needle tending to turn the needle at right angles to the wire. The amount the needle is turned will depend upon the value of the current in the wire. There is a definite relation between the direction of the current in the wire and the direction of the magnetic field surrounding the wire, because a reversal of current in the conductor will result in a reversal in the direction in which the compass needle is deflected. Remembering that the direction of a magnetic field can be determined by placing a magnetic needle in the field and noting the direction in which the N-pole of the needle points, this being taken as the positive direction, if one looks along a conductor in which there is a current and the current be from the observer, the direction of the magnetic field about the conductor will be clockwise. Imagine a conductor carrying a current and that you are looking at a cross-section of this conductor (see Fig. 3), and the direction of the current in the conductor is from you (this being indicated in the figure by the cross inside the circle), then the lines of force of the magnetic field will be concentric circles about the conductor, they being nearer together near the conductor, indicating the strength of the field is greatest near the conductor. A compass needle placed above the conductor would place itself in such a position that the N-pole would point toward the right and the S-pole toward the left. If the needle be placed below the conductor, the N-pole would point to the left and the S-pole to the right, indicating that the direction of the magnetic field above the conductor is just the reverse of what it is below the conductor.
Fig. 3—Lines of Force
Fig. 4—Reversed Lines of Force
The strength of the magnetic field produced by a current in a conductor can be greatly increased by forming the conductor into a coil. Figure 4 shows the cross-section of a coil composed of a single turn of wire. The current in the upper cross-section is just the reverse of what it is in the lower cross-section, as indicated by the cross and dash inside the two circles. As a result of the direction of current in the two cross-sections being different, the direction of the magnetic field about these two cross-sections will be different, one being clockwise, and the other counter-clockwise. It will be observed, however, that all the lines of force pass through the center of the coil in the same direction, or the magnetic field inside the coil is due to the combined action of the various parts of the conductor forming the complete turn. This magnetic field can be increased in value, without increasing the current in the conductor, by adding more turns to the coil.
Fig. 5—Magnetic Lines Passing through Center
A cross-section through a coil composed of eight turns placed side by side is shown in Fig. 5. The greater part of the magnetic lines created by each turn pass through the remaining turns as shown in the figure, instead of passing around the conductor in which the current exists that creates them. This results in the total number of lines passing through the coil per unit of cross-sectional area being greater than it was for a single turn, although the value of the current in the conductor has remained constant, the only change being an increase in the number of turns forming the coil.