It's interesting you should say that. For a long time I've predicted that the speed of light thru a charged capacitor (say vacuum dielectric, or any other) should be slower than normally. Has this experiment been done? According to my interpretation of Dewey B. Larson's Reciprocal System theory, it should. That would be easy to test -- just shine a laser between the plates of a charged capacitor at an angle to a linear edge and see if there's an offset in the emerging beam.
See: http://www.rstheory.com/ http://www.reciprocalsystem.com/ Frederick Sparber wrote: ...marginally slower rate of time passage, which means, from the point of view of an outside observer, C should be slightly lower between the plates.