Electrodes are used for recording several different biological signals. These include electro-cardiograph, electro-myograph, electro-encephalograph and others. In general, the electrode impedance varies as a function of electrode current and frequency. The linear model is an approximation that assumes that the electrode impedance does not change by electrode current, and the frequency dependency of the impedance is that of a first-order system, and thus it can be modeled with fixed electronic components. The half-cell potential is usually not much of an interest to the experimenter because AC amplifiers are used with most biological signals (except electro-oculogram – EOG) and the half-cell potential of an electrode varies a lot depending on the electrolyte gel and the temperature. In hospitals the clinical equipment has isolated grounds that prevent the passage of AC or DC current from such equipment to the patients. Isolated grounds are a safety measure to protect patients in case of faulty equipment.