In this paper, a square coil system similar to the Merritt four coil system is investigated. This system is used for a biomagnetic experiment at the laboratory at the Department of Biology, Faculty of Sciences, University of Novi Sad. For the experiment, it is necessary that the magnetic field inside the coils be uniform. Therefore, distribution of the magnetic flux density inside the coil system is considered. For this system, magnetic flux density produced by sinusoidal current through the coils is calculated and then uniformity of the magnetic field inside the coils is investigated. Calculated magnetic flux density is compared to the measured magnetic flux density obtained experimentally at the laboratory. .