In _Us Conductors_ (page 188) Sean Michaels describes a special superior instrument that Theremin has built just for the love of his life, Clara.
As Theremin, Michaels writes about being in the audience and the orchestra tuning, and the instrument that gives the A is not the oboe, but her theremin - he says, he knows there is a lamp on her side of the cabinet that glows when she's making a "perfect A."
I wonder how he did this? A crystal? A tuning fork with a sensor? Because an ordinary RC circuit would not ever keep a pitch, eh? I suppose it could have been an RC circuit that she tuned with a tuning fork just before the performance. Hmmm.
Also why doesn't MY theremin have a Perfect A Lamp?