Figure 2

Electrical schematic diagram of the measurement setup. The 16 discs inside the sample area represent the spring-loaded electrodes of Fig. 1. The distance between nearby contacts along an edge is 2 mm, while the distance between nearby contacts at a corner is 2.12 mm. The dash-dotted box represents the switching unit, which includes 4 × 8 relay switches (only a few are drawn). A full (empty) dot represents a closed (open) relay switch. In the present setup, the dc voltmeter is a Keysight 34461A; the dc ammeter is a Keithley K2602B; the switching unit is a Keysight 34933A reed relay matrix in a 34480A host. All instruments are interfaced through an IEEE-488 bus.