Theremin later built a third model after his return to the Soviet Union toward the end of the 1930s , which now resides at the Theremin Center in Moscow , though it is not currently operational .
Theremin later built a third , much smaller model after his return to the Soviet Union toward the end of the 1930s . 
 This version of the instrument now resides at the Theremin Center in Moscow , though it is not currently operational .