Oxidation Reduction Potential (ORP) electrode is used to measure oxidize contaminants. An ORP probe is really a mill volt meter, measuring the voltage across a circuit formed by a reference electrode constructed of silver wire (in effect, the negative pole of the circuit), and a measuring electrode constructed of a platinum band (the positive pole), with the pool water in between. The reference electrode, usually made of silver, is surrounded by salt (electrolyte) solution that produces another tiny voltage. But the voltage produced by the reference electrode is constant and stable, so it forms a reference against which the voltage generated by the platinum measuring electrode and the oxidizers in the water may be compared. The difference in voltage between the two electrodes is what is actually measured by the meter. Modern ORP electrodes are almost always a combination electrode that is both electrodes are housed in one body - so it appears that it is just one probe. Incidentally, the meter circuitry itself must have very high impedance (resistance) in order to measure the very tiny voltages generated by the circuit.