In this experiment, the students investigate the relationship between the current I and the magnetic flux density B for 2 coils in a DC circuit. The aim is to check whether the formula for calculating the magnetic flux density B of long air coils can also be used for coils such as those used in this experiment, i.e. multi layer, short coils suitable for student experiments. In the additional tasks, the magnetic flux density B is calculated using a much more complex formula. Although this applies to cylindrical coils, it provides very good results compared to the measured values in this experiment.