The second signal that is found on the electrode is coming from the mains, 220 V / 50 Hz in most of the European countries, 110 V / 60 Hz in many others.
Suppose the body is completely floating and the capacitive coupling to the mains and to the earth ground is equal. Then the potential of the body really is 110 V with reference to the earth ground. This is the reason that TMSi amplifiers always use a floating measurement system, which means that the zero of the power supply of the measurement system is floating on the same 110 V. If there is a moderate difference between the zero of the power supply and the potential of the human body, the amplifier cannot amplify the signal anymore, the signal will be out of range. For this reason, the zero of the power supply is connected to the human body so that under most circumstances the signals will be in range.
In that case, the potential of the body will only slightly differ from the zero potential of the power supply of the amplifier, both with reference to earth ground. This small difference is called the “Common Mode” signal, and is in the range of 0-100 mV, and the frequency will be 50 or 60 Hz of course. Common mode means that this signal is available on all the electrodes, and is defined as the mean of all the electrode signals.
The mains interference part of the common mode potential is caused by the leakage current between the body and the zero of the amplifier, where the amplitude depends on the leakage current itself, and of the impedance of the electrode, that the leakage current is flowing through.
But the mains interference sometimes is not a common mode signal at all. The cables between the electrode and amplifier input are capacitively coupled to the mains as well. If the coupling is not exactly the same for all the cables, interference current will flow between the cables from one electrode to another, causing a differential mains interference distortion. This means that the mains interference will always be a combination of several distortions, and different solutions must be used to get rid of it. In the figure below, an example of 50 Hz noise is shown.