  It is generally impossible to separately measure the resistance of the
functional component (i.e., the intrinsic device materials) and the parasitic
component (i.e., terminals, interfaces and serial loads) in a two-terminal
device. Yet such knowledge is important for understanding device physics and
designing device systems. Here, we consider a case where an electric current,
temperature, or magnetic field causes a small but identical relative
conductivity change of the device materials. We find an exact solution to this
relative change by a simple resistance-data analysis of similarly configured
two-terminal devices. The solution is obtainable even if the change is quite
small, say, less than 0.1%. In special cases of small relative changes in
parasitic resistance, the absolute parasitic resistance is also obtainable. Our
method is especially useful for studying the switching and transport
characteristics of the emergent non-volatile resistance memory.
