Abstract: An electric current comprising a plurality of harmonic components is passed through the pipe under test, a magnetic field produced by it outside the pipe is measured, a spectrum of the electric current is measured, a spectrum from measured magnetic field data is also obtained. The ratios between identical spectral components of the current and magnetic field are evaluated. The test apparatus comprises a current source supplying an electric current, a means for measuring and storing momentary current values, a means for evaluating a current spectrum to define harmonic components of the electric current, at least one magnetic field sensor for measuring and storing momentary values of the magnetic field produced outside the pipe by the current, a means for evaluating the magnetic field spectrum to define harmonic components of the magnetic field, and a means for comparing and analyzing the obtained spectral data of the current and magnetic field.