The dapper, young Maurice Martenot in his early twenties.
The equally striking, tuxedo clad Leon Theremin, in his early 30's.
Leon Theremin's assistant, Julius Goldberg, is experimenting in the above photo with the "etherphone", a precursor of the theremin. The picture was taken in Germany in 1927, prior to Leon Theremin's arrival in America. The device is controlled by a foot pedal (volume) and push button articulation regulators. Pitch is determined by proximity to the vertical antenna, just like modern theremins. This sort of prototype theremin was essentially the same as the theremin we are familiar with today except that it was provided with an audio cutoff switch (or "articulation regulator") controlled by the left hand. This cutoff switch offered the player the possibility of playing rapid "staccato" notes on the instrument. The German innovator, Martin Taubman, following the public demonstration of the etherphone at an exhibition in Hamburg in 1927, experimented further with the device and adapted it so that the volume cutoff could be held in the left hand of the player. Taubman called his version of the etherphone the "electronde" and you can hear a short mp3 of what it sounded like by clicking on the link at the end of this paragraph. Any modern theremin can be easily adapted to include a cutoff switch - either incorporated into a box (like the etherphone) or hand held (like Taubman's electronde). If you would like to hear what the Etherphone sounded like, here is a short mp3 I made of a modern theremin with a simple, hand-held audio cutoff switch similar to the kind of thing used in the etherphone and electronde.
The techniques of instrument control that were experimented with by Maurice Martenot are very similar to those investigated by Leon Theremin. There is an interesting comparison to be made between the photo of Martenot (above) and that of Julius Goldberg. The difference, of course, is that Maurice Martenot is attached to his "ondes" by means of a ring and a wire with which he changes the pitch, while Julius Goldberg is using only proximity to the vertical antenna to control pitch. A closeup of the "ring" that fits onto the finger of the ondiste can be seen in the inset.