I was totally astonished by the beauty of the sound of the instrument, called a Theremin! I've never seen anything like this before, the intstrument sounds like a violin or the voice of an opera singer.
The instrument was designed in 1919 by Léon Theremin and became very popular. The woman you see in the video, Clara Rockmore, was one of his pupils. The Theremin is an instrument with two antennes and your hands have to move between those two antennas. The right hand affects the pitch of the tone and the left hand affects the volume. Because it reacts very easily to movements, it is a very expressive instrument.
Also Jimmy Page from Led Zeppelin played the theremin in his song Whole lotta love. It is the weird ufo-like sound when you hear when Robert Plant is singing his ' ah ah ah ah ah ah ah'
And here you see old Jimmy Page playing the Theremin: