When something occurs at one point in space because something else happened at another point, with no visible means by which the "cause" can be related to the "effect," we say the two events are connected by a field. In radio work, the fields with which we are concerned are the electric and magnetic, and the combination of the two called the electromagnetic field.

A field has two important properties, intensity (magnitude) and direction. The field exerts a force on an object immersed in i t ; this force represents potential (ready-to-be-used) energy, so the potential of the field is a measure of the field intensity.

The direction of the field is the direction in which the object on which the force is exerted will tend to move. An electrically charged object in an electric field will be acted on by a force that will tend to move it in a direction determined by the direction of the field.

Similarly, a magnet in a magnetic field will be subject to a force. Everyone has seen demonstrations of magnetic fields with pocket magnets, so intensity and direction are not hard to grasp.

A "static" field is one that neither moves nor changes in intensity. Such a field can be set up by a stationary electric charge (electrostatic field) or by a stationary magnet (magnetostatic field). Rut if either an electric or magnetic field is moving in space or changing in intensity, the motion or change sets up the other kind of field.

That is, a changing electric field sets up a magnetic field, and a changing magnetic field generates an electric field. This interrelationship between magnetic and electric fields makes possible such things as the electromagnet and the electric motor.

It also makes possible the electro-magnetic waves by which radio communication is carried on, for such waves are simply traveling fields in which the energy is alternately handed back and forth between the electric and rnagnetic fields.

Lines of Force
Although no one knows what it is that composes the field itself, it is useful to invent a picture of it that will help in visualizing the forces and the way in which they act. A field can be pictured as being made up of lines of force, or flux lines.

These are purely imaginary threads that show, by the direction in which they lie, the direction the object on which the force is exerted will move. The number of lines per unit of area (square inch or square centimeter) is called the flux density.

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