While the Ondes Martenot is one of the oldest electronic musical instruments, it is still played today. It is known for making extremely expressive electronic sounds. A major advantage of the Ondes Martenot is that it enables players to control the temporal dynamics of notes in a particularly sensitive manner by pressing an "intensity key". In this paper, the intensity key is characterized experimentally. Measurements were carried out on the force with which the musician presses on the key, the depression of the key, and the resulting sound. It is demonstrated that this key provides a broad dynamic range of 50 dB for each note over the entire range of the instrument. Moreover, the change in sound intensity of the notes depends only on the displacement of the intensity key and then on the force applied on it. Mechanical parts of the intensity key were made for the musician to control finely and smoothly the variations of the sound produced by the instrument. A mechanical model is developed to explain experimental results. It appears that fine control is related to the mechanical behavior of the key: it acts as a well-chosen non-linear spring. It shows a linear relationship between the logarithm of the finger force on the key and the dBSPL (sound pressure level) increase.