The circular logo at the top of each page represents a circular conductive object with 16 electrodes (the spokes) equally spaced around the edge. If an electric current is applied between adjacent electrodes (at 11 and 12 o'clock) and the resulting voltage measured on other pairs (in this case between 3 and 4 o'clock), it can be assumed that the measurement is sensitive to impedance changes in the brown wedge. This is the principle behind one of the most successful EIT systems to date, the Sheffield Mark 1 system.