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Why are ammeters ineffective for testing resistance between structures in an electrolyte?

  1. They require a direct connection to function

  2. There is often a voltage difference between isolated structures

  3. They provide inaccurate readings

  4. They cannot measure low resistances accurately

The correct answer is: There is often a voltage difference between isolated structures

The reason that ammeters are ineffective for testing resistance between structures in an electrolyte primarily revolves around the presence of a voltage difference between isolated structures. When testing resistance, especially in a scenario involving cathodic protection systems, it is essential to understand that ammeters measure current flow, which can be significantly influenced by potential differences in the environment. If there is a voltage differential between two isolated structures, the current that flows through the ammeter will not solely be representative of the resistance between those structures; rather, it may include contributions from other factors such as ground potential rise or other miscellaneous electrical paths. This can lead to misleading or inconclusive readings regarding the actual resistance present in the electrolyte. Furthermore, for accurate resistance measurements, a method that isolates the structures and eliminates unintended current paths would be more suitable, such as using a micro-ohmmeter or other resistance-specific instruments. Thus, while measuring resistance, voltages between structures must be accounted for, which is where the limitation of using ammeters becomes apparent.