A [[curve]] recorded as a result ot simultaneous SP measurements from two [[electrode|electrodes]] located downhole, each serving as a reference [[potential]] for the other. The purpose of a differential SP is to minimize or eliminate unwanted effects induced on the SP measuring circuitry (i.e., electrode and [[survey]] conductor). The "twin" circuitry allows spurious efforts induced simultaneously on each segment of the circuit to negate each other at the recording [[galvanometer]]. The two downhole electrodes selected for this purpose should be separated by enough distance that their equal and opposite responses do not cancel the valid SP [[signal|signals]] produced by different segments of the formation. See [[SP]], [[downhole ground]], and [[telluric currents]].