Magnetic flowmeters, also known as electromagnetic flowmeters or induction flowmeters, obtain the flow velocity by measuring the changes of induced voltage of the conductive fluid passing across a controlled magnetic field.
A typical magnetic flowmeter places electric coils around (inline model) / near (insertion model) the pipe of the flow to be measured and sets up a pair of electrodes across the pipe wall (inline model) or at the tip of the flowmeter (insertion model). If the targeted fluid is electrically conductive, i.e., a conductor, its passing through the pipe is equivalent to a conductor cutting across the magnetic field. This induces changes in voltage reading between the electrodes. The higher the flow speed, the higher the voltage.
Image courtesy of TÜV NEL