The iTEV 90 offers the option for use of a third current electrode which can be used for various purposes:

Extracellular field compensation

Use for extracellular field compensation by injecting a fraction of the measured current at a point outside the cell at same distance to the cell membrane as the current electrode which is impaled the cell. The compensation clamp controller is operated in current clamp mode. As shown by Baumgartner et al. (Biophys. J. 77:1980-1991, 1999) such a current injection reduces spatial and temporal inhomogenities in the membrane potential upon applying a voltage pulse.

Intracellular current injecting electrode

Use as additional intracellular current injecting electrode. The compensation clamp controller is operated in voltage clamp mode with the command voltage as input signal. The advantage of this configuration is that the current is injected more uniformly within the oocyte and that the overall access resistance is reduced by about a factor of 2. Both effects allow an increase in clamp speed. In addition, the current electrode tips (and the oocyte) pass a smaller peak current resulting in a longer life time of tip and oocyte, providing more long-term stability of the recording.