Most electrochemical work with an electrochemical cell is achieved using what is called a potentiostat. A potentiostat is an electronic device that controls the voltage difference between a working electrode and a reference electrode. Both electrodes are contained in an electrochemical cell. The potentiostat implements this control by injecting current into the cell through an auxiliary electrode. In almost all applications, the potentiostat measures the current flow between the working and auxiliary electrodes. The controlled variable in a potentiostat is the cell potential and the measured variable is the cell current. A potentiostat typically functions with an electrochemical cell containing three electrodes and that is true for both field probes and lab cells (reference):
Working Electrode: Electrochemical reactions being studied occur at the working electrode. In corrosion testing, the working electrode is a sample of the corroding metal. Generally, the working electrode is not the actual metal structure being studied. Instead a small sample is used to represent the structure. This is analogous to testing using weight loss coupons. The working electrode can be bare metal or coated.
Reference Electrode: A reference electrode is used in measuring the working electrode potential. A reference electrode should have a constant electrochemical potential as long as no current flows through it.
Auxiliary Electrode: The Auxiliary electrode is a conductor that completes the cell circuit. The auxiliary (counter) electrode in lab cells is generally an inert conductor like platinum or graphite. In field probes it's generally another piece of the working electrode material.
Electrometer: The electrometer circuit measures the voltage difference between the reference and working electrodes. Its output has two major functions: it is the feedback signal in the potentiostat circuit and it is the signal that is measured whenever the cell voltage is needed. An ideal electrometer has zero input current and infinite input impedance.
I/E Converter: The Current to Voltage (I/E) converter in the simplified schematic measures the cell current. It forces the cell current to flow through a current measurement resistor. The voltage drop across that resistor is a measure of the cell current.
Control Amplifier: The control amplifier is a servo amplifier. It compares the measured cell voltage with the desired voltage and drives current into the cell to force the voltages to be the same. Under normal conditions, the cell voltage is controlled to be identical to the signal source voltage.
The Signal: The signal circuit is a computer controlled voltage source. It is generally the output of a Digital to Analog (D/A) converter that converts computer generated numbers into voltages.
Galvanostats and Zero Resistance Amplifiers (ZRAs): Most laboratory grade potentiostats can also be operated as galvanostats or ZRAs.