The theory for the current-sensing electromagnetic flowmeter was newly developed. The current-sensing flowmeter can achieve the measurement with a high temporal resolution so that it can be applied to measure the flows with fast transients like two-phase flow. The signal prediction and the calibration of the current-sensing flowmeter in simplified two-phase flow were conducted, and the given calibration process would be an important step toward the calibration for real two-phase flow. The three-dimensional virtual potential distributions for the electrodes of finite size were computed for single-phase flow, annular flow, and modeled slug flow. With the gradient of the virtual potential, weight functions related to each flow pattern were deduced. A flow pattern coefficient f was introduced to simplify the calibration process for two-phase flow and measured with the impedance spectroscopy method. In order to measure the local mean velocity of a developing flow using the electromagnetic flowmeter, a localization parameter was modeled and compared with experimental data.