"
 I have a 1949 edition of the GE ""Manual of Electric Instruments: Construction and Operating Principles"" which describes a 4-quadrant power factor meter of the polarized vane type - it works like a miniature synchronous motor, with a polyphase rotating field like a motor, energized with the phase voltages, and the rotor magnetized by one selected phase current; the angle of the rotor directly represents the angle between voltage and current. The book describes, but sadly doesn't give a picture of, a 360 degree 4-quadrant scale, said to be useful for tie lines where the source and load ends can interchange. The GE manual says to use a wattmeter and varmeter for any critical measurements, because a single-current-coil power factor meter produces meaningless results in an unbalanced system.   "