One of the first radio-electric instrument is called 'Theremin' invented 1920s. It has two antennas, one for change the tone, the other for change the volume. Very simple signal is produced by oscillators by the position of hands, right hand for tone, left for volume. They are changed by how close your hands are to the antennas. The sound of Theremin is very simple. Although if it is used with effecters, you can get very unique sounds.
My theremin has only one antenna for tone, no volume control by bought at Ishibashi Music Shop. The original Theremin looks like a bust box though, mine are looks like a guitar effecter. Some Theremin players can control the tone by moving their hands to produce fancy music, though it is very difficult.
This is my Theremin. It's compact version of Theremin. If you are Theremin freak, you don't want it call a Theremin?
Compare to cigarette box next to Theremin. It is the almost same size of single effecter for guitar/bass.
Now, look inside of Theremin. I took this picture, because I fell the Theremin to the floor and the case has broken. So I can open it without drivers.
There are two knobs to control Theremin. One for Volume (Upper side), the other for frequency adjustment (Lower side).
Moving hands around the antenna. This is the end of story. The person of the left picture is MARIO; an infamuse dancer, not me.