All the voltage drops in the cathodic protection circuit are controllable except for the one through the:

In a cathodic protection system, the voltage drops that occur within the circuit can be influenced or adjusted except for the one that occurs through the electrolyte. The electrolyte, which is typically the soil or water surrounding the metallic structures being protected, provides the medium through which current flows.

The resistance of the electrolyte is determined by factors such as its conductivity, temperature, and composition, which are not easily modified or controlled directly by operators. While other components of the system, such as the metallic path, anodes, and cathodes, can have their properties adjusted (e.g., by using different materials or changing configurations), the electrolyte's resistance remains largely constant and is a fundamental characteristic of the environment in which the protection system operates.

Since the voltage drop across the electrolyte is largely due to its inherent properties, and since it dictates how current flows within the protection system, it is considered the uncontrollable aspect in the circuit compared to the other components where modifications can be made to influence performance.