What measures the amount of magnetic flux density in a fixed area?

Magnetic flux density refers to the amount of magnetic flux passing through a unit area in a magnetic field. It is a measure of how strong the magnetic field is at a specific point in space. In a fixed area, magnetic flux density is typically expressed in teslas (T), which can be calculated by taking the magnetic flux (measured in webers) and dividing it by the area (measured in square meters).

This concept is foundational in physics and engineering, especially in understanding how magnetic fields interact with materials and devices, such as in transformers and electric motors. Knowing the magnetic flux density helps predict how materials will respond to magnetic fields, which is crucial for designing effective electromagnetic systems.

The other options, while relevant to the broader context of electromagnetism, do not specifically measure magnetic flux density in a fixed area. Voltage relates to electric potential differences, magneto-resistance deals with how a material's resistance changes in a magnetic field, and field strength is a different characteristic that describes the intensity of the magnetic field at a point but doesn't quantify the flux through a specific area. Thus, magnetic flux density directly addresses the measurement in question.