Two sets of measurements need to be made: one with a magnetic field in the positive "z"-direction as shown above, and one with it in the negative "z"-direction. From here on in, the voltages recorded with a positive field will have a subscript P (for example, "V13, P" = "V3, P" - "V1, P") and those recorded with a negative field will have a subscript N (such as "V13, N" = "V3, N" - "V1, N"). For all of the measurements, the magnitude of the injected current should be kept the same; the magnitude of the magnetic field needs to be the same in both directions also.