In physics lessons at secondary school and experimental physics courses at universities, the magnetic field inside a current-carrying solenoid is considered quantitatively. The corresponding equations and theories are supported by measuring the magnetic flux density inside the solenoid with a Hall probe. It has already been shown that smartphones with their internal magnetometer are suitable for quantitative investigations of magnetic fields. E.g., Silva proposed to wind a small coil on a pen and investigate the dependence of the stray field on the number of windings. Ogawara et al. placed a smartphone inside a current loop and studied the dependence of the flux density on current "I."