Compasses are included in many devices, from satnav and mobile phones to mapping systems and rangefinders. Accurate calibration can improve the accuracy of the data obtained from these compasses. Helmholtz Coils are perfect for precise calibration of any type of vector magnetometers and some scalar magnetometers, whether in a device or a stand-alone sensor. The PA1 and CU1 enable a coil to be controlled from a computer with a suitable data acquisition card. The CU2 module enables active compensation of slowly varying ambient field changes.