A device for and a method of outputting a magnetic field which is selected according to the type of card reader into which the device is inserted. Different card readers have different reader head types and different number of reader heads, and different modes of outputting the magnetic field, usually with the same information contents, are selected in order to cater for different reader types. In one example, the sensing coils of different reader heads may have different angles to the swipe plane, so that different field line angles may be selected to adapt to the actual reader head.