What is the basic operating principle of electrodynamic movement?

The basic operating principle of electrodynamic movement involves the interaction of electromagnetic forces to produce motion, specifically in devices like analog voltmeters and ammeters. In this principle, the use of fixed coils instead of permanent magnets allows the movement to be influenced by the current flowing through the coils. When the electric current flows through these fixed coils, it generates a magnetic field that interacts with the movement of a coil armature, leading to rotation or deflection on a scale.

This principle is essential because it enhances the sensitivity and accuracy of the measuring instrument, allowing it to operate effectively in a variety of conditions. By using fixed coils, the design also benefits from greater stability and reduced mechanical wear over time, compared to systems relying on permanent magnets which can lose strength or become misaligned.

The other options do not accurately describe the fundamental operating mechanism of electrodynamic movement. For instance, the reliance on a fluid medium for measurement corresponds more closely with different types of measuring instruments that utilize hydraulic principles, and operating without any moving parts generally describes a different category of technology such as capacitive or resistive sensors. Measuring resistance via thermal effects pertains to thermistors and similar devices, which do not align with the electrodynamic movement principle.