1. IN A BALANCING APPARATUS ADAPTED TO BE CONNECTED TO A PAIR OF PICKUPS FOR TWO PLANE BALANCING OF A ROTATING ROTOR, A REFERENCE OPERATOR CONNECTABLE TO AT LEAST ONE OF SAID PICKUPS TO BE ENERGIZED THEREBY TO PRODUCE A REFENCE SIGNAL OF FIXED AMPLITUDE AND ADJUSTABLE PHASE, A PAIR OF COMPENSATOR OPERATORS CAPABLE OF ADJUSTING THE PHASE AND AMPLITUDE OF A SIGNAL FED THERETO, SAID REFERENCE OPERATOR BEING CONNECTABLE TO EACH OF SAID COMPENSATORS OPERATORS, A PAIR OF MIXERS FOR MIXING THE OUTPUTS OF SAID PICKUPS AND THE OUTPUTS OF SAID COMPENSATOR OPERATORS, MEANS FOR CONNECTING THE OUTPUT OF SAID REFENCE OPERATOR TO SAID COMPENSATOR OPERATORS, AND MEANS FOR CONNECTING ONE OF SAID PICKUPS AND THE OUTPUT OF ONE OF SAID COMPENSATOR OPERATORS TO ONE OF SAID MIXTERS AND FOR CONNECTING THE OTHER PICKUP AND THE OTHER COMPENSATOR OPERATOR TO THE OTHER OF SAID MIXERS TO PRODUCE OUTPUT SIGNALS FROM SAID MIXERS WHICH SIMULTE THOSE WHICH WOULD BE OBTAINED FROM A BALANCE ROTOR.