Magnetic Field

Author: E.E. Kimberly

A bar of hard steel that has been magnetized is surrounded by a space within which other magnets are attracted or repelled by it. This space is called the magnetic field and is theoretically infinite in volume. The force with which a field acts upon another magnet at any point is dependent on the intensity of the field at that point. The intensity at any point in a field is dependent on its distance from the magnet which produces the field. The intensity of a magnetic field at any point is defined in terms of the force which it would exert upon a unit pole placed at that point. In the concept of unit pole, it is assumed that either an isolated north pole or an isolated south pole of a magnet can be produced in space. If two such isolated poles be placed 1 centimeter apart and their magnetic strengths be made equal and of such a value that they attract or repel one another with a force of 1 dyne, then each pole is said to have the strength of 1 unit pole.

By definition, a magnetic pole which is attracted toward the north by the earth's magnetic field is a north pole, and one which is attracted toward the south is a south pole. If a magnetic compass be brought into the proximity of a bar magnet, as in Fig. 1-1, the compass needle will assume a definite direction.

As the compass is moved about in the field, it will be seen that the direction of the force acting on the needle changes from point to point. At every point the magnetic field has two defining properties, intensity and direction.

If the compass be moved always on a line drawn longitudinally through the needle, it will describe a smooth curve in space between the north and south poles of the magnet. This curve is said to represent the path of a magnetic line of force. For quantitative purposes the strength of a magnetic field at any point is defined in terms of lines of force or maxwells per square centimeter. Thus, if a unit pole in a magnetic field be acted upon by a magnetic force of 1 dyne, the field intensity at that point is said to be 1 line of force per square centimeter or 1 gauss.

Fig. 1-1. Magnetic Field Around a Bar-Type Magnet

All lines of force are said to "emerge" from the north pole and to "enter" at the south pole of a magnet and to close upon themselves within the magnet. Fig. 1-2 shows the magnetic field produced by a horseshoe magnet. Properties of the magnetic field are further discussed in Chapter 7.

Fig. 1-2. Magnetic Field Around the Poles of a Horse-shoe-Type Magnet