4. Vacuum Tubes
If you thought the theremin was a weird instrument, then the ondes Martenot will really freak you out. Cousin to theremin, the ondes also has an eerie sound and functions by varying electrically-generated wave frequencies. But it departs from its antennaed brethren in its method of being played: by slipping your finger in a ring that is attached to a string and moving your hand along the length of the string, which vary the oscillation frequency of sound waves generated in a vacuum tube.
A vacuum tube is a vacuum-sealed container through which an electric current runs. That current is generally created by a filament, like a light bulb, and early vacuum tubes in fact evolved from incandescent light bulbs. Through an effect called heterodyning, two high-frequency radio waves - inaudible to the human ear, of course - combine to create an audible wave of a frequency equal to the difference between the two radio waves.
As with the theremin, the human body affects the capacitance of the resulting electromagnetic system and thus affects the frequency of the sound wave. But rather than have you wave your hands around like a crazy person, the ondes has you tether your hand to a string that runs the length of a keyboard by slipping your finger in a ring. The keys could either be played directly to produce fixed notes, or they could be used as note markers for when the ondes is played by string.
While the right hand is on string duty, the left hand operates a control button that determines volume, similar to the theremin. When the button isn't pressed at all, the ondes makes no sound. The harder the button is pressed, the louder the sound, and the manner in which you press the button determines the attack of the note.