The ondes Martenot is an electric musical instrument that was presented for the first time in Paris in 1928. An electrical waveform is generated by a heterodyne oscillator whose fundamental frequency is controlled by modifying the oscillator capacitance/inductance using a ribbon/keyboard. A drawer enables the performer to filter the wave and to specify the volume and articulation. The resulting wave is fed to one or multiple diffuseurs, that convert it into soundwaves. The first diffuseurs used a so-called electromagnetic (or moving-iron) loudspeaker, that was combined around 1937 with an electrodynamic (or moving-coil) loudspeaker, which is the standard technology today. The behaviour of a less known electromagnetic loudspeaker from 1937 is studied with the longer term goal to preserve the sound heritage of an instrument whose conservation state does not allow for musical practice. The loudspeaker consists of a permanent magnet and two electromagnets whose combined flux lets a metal reed vibrate. A rod connects the moving reed to the apex of a paper cone, that generates the resulting soundwave. In the current paper, a minimal physical model for this electromagnetic loudspeaker is derived. It is shown that its operating principle is inherently nonlinear, as opposed to the linear Thiele & Small model for electrodynamic loudspeakers. Furthermore, a first experimental measurement on a heritage diffuseur enables to quantify the nonlinearities presented by the electromagnetic loudspeaker.