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In the 1840s, Michael Faraday proposed the idea that invisible lines of force are at work in electrical currents and magnetism. These hypothetical lines make up a force field that has a certain value and direction at any given point and can be used to calculate the total force acting on a particle at that point. This concept was quickly adapted to apply to gravity in the form of a gravitational field.

According to Faraday, these invisible lines of force are responsible for electrical force (as shown in Figure 5-6) and magnetic force (as shown in Figure 5-7). They result in an electric field and a magnetic field that can be measured.

FIGURE 5-6: Positive and negative charges are connected by invisible lines of force.

FIGURE 5-7: The north and south poles of a bar magnet are connected by invisible lines of force.

Faraday proposed the invisible lines of force, but he wasn’t nearly as clear on how the force is transmitted, which drew ridicule from his peers. Keep in mind, though, that Newton also couldn’t fully explain how gravity is transmitted, so there was precedent to this. Action at a distance was already an established part of physics, and Faraday, at least, was proposing a physical model of how it could take place.

The fields proposed by Faraday turned out to have applications beyond electricity and magnetism. Gravity, too, can be written in a field form. The benefit of a force field is that every point in space has a value and direction associated with it. If you can calculate the value of the field at a point, you know exactly how the force will act on an object placed at that point. Today, every law of physics can be written in the form of fields.