Final answer:
The FM 3-25-26 definition of a grid-magnetic angle relates to the difference between grid north and magnetic north in navigational terms. It is not about electromagnetic interactions within coils and antennas, although magnetic field concepts are tangentially related.
Step-by-step explanation:
The FM 3-25-26 definition of a grid-magnetic angle is related to land navigation and the use of maps, which falls under the subject of Geography, particularly in the context of military field manuals. In such manuals, the grid-magnetic angle is typically the difference between grid north (as shown on a map's grid lines) and magnetic north (as shown by a magnetic compass).
A relevant concept to this definition, though not specifically the grid-magnetic angle, is how a magnetic field interacts with objects in the field. For example, when a coil of wire, such as a 200-turn flat coil that is 30.0 cm in diameter, is exposed to a magnetic field with a strength of 1.00×10-¹² T perpendicular to the coil, it can induce an electromotive force. This principle is often used in the design of antennas for FM radios.
In terms of magnetic field dynamics, the magnetic field strength is illustrated as directional lines around a magnetic material, indicating the direction and magnitude of the field. When there is a change in magnetic flux over an area A at an angle with the perpendicular to this area, it can induce an electromotive force or current in the conductor. This magnetic interaction is essential to many engineering applications and is also an interesting physical phenomenon for studies in physics.