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Fig. 1 | Earth, Planets and Space

Fig. 1

From: A procedure for stable electrical measurements on a rock sample against high contact resistance as a prerequisite for electrical tomography

Fig. 1

Circuitry of a the conventional four-terminal method and b the differential measurement method adopted here. RIN is input resistance, RV is the insulation resistance between the negative terminal and chassis ground, Rc is the contact resistance between the electrode and sample surface, and Rs is the sample resistance. HI labels positive terminals, LO labels negative terminals, C1 and C2 are current electrodes, P1 and P2 are potential electrodes. In a when the sum of Rs3 and Rc2 is much smaller than RIN or RV, no current flows into the voltmeter. However, in a voltmeter, RV is usually much smaller than RIN. When RV is less than the sum of Rs3 and Rc2, current flows into RV through the negative terminal of the voltmeter, preventing correct voltage measurement. In b, RV does not contribute to the current path, and when RIN is larger than the sum of Rs3 and Rc2, most of the current does not flow to the voltmeter. The differential method uses two voltmeters, whose positive terminals are connected to P1 and P2, and whose negative terminals are shorted to the negative terminal of the current source. The voltage between P1 and P2 is obtained as the voltage difference between the measured values of V1 and V2. In addition to this circuitry, the guarding to reduce current leakage is adopted as described in the main text; however, the shield is not drawn in this figure to avoid complexity of the diagram. See, for example, Figs. 2–5 and Fig. 2–39c of Tektronix (2016) for a detailed explanation of the guarding

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