This instrument use a moving-iron rationmeter, with eddy-current damping. Only when the pointer stops on the scale mark, the frequencies and phase angles of the voltages of both generatos are the same. If it stops on another point, there is phase displacement between both voltages or at least one generator is disconnected. When the frequency difference is less than approximately 1.5Hz, the pointer rotates in the direction marked as “+” (if the frequency of the generator G2 is higher than the frequency of G1) or in the direction marked as “-” (if the frequency of G2 is lower).