C.sub.X is measured in the presence of R.sub.P by charging C.sub.X with a constant current I.sub.C for a period time and taking four (time, voltage) pairs: (t.sub.A,V.sub.A), (t.sub.B,V.sub.B), (t.sub.C,V.sub.C) and (t.sub.D,V.sub.D). The first pair are sufficiently close together that the short straight line segment connecting (t.sub.A, V.sub.A) and (t.sub.B, V.sub.B) is not significantly different from the corresponding segment of a true graph of the capacitor's actual charging. A similar requirement is imposed on the time interval between t.sub.C and t.sub.D. The time interval t.sub.At.sub.B need not be the same as t.sub.Ct.sub.D. The interval t.sub.Bt.sub.C should be long enough that if there is significant R.sub.P present then there is at least a discernable difference between the true graph and a straight line between the first pair and the second. C.sub.X and R.sub.P are computed as a function of the four pairs and I.sub.C, and C.sub.X may then be diminished by a previously measured value for a stray capacitance present in the absence of C.sub.X.