Electronic Analog and hybrid computers, Korn and Korn.
Test results for: Hamlin SPST reed relay, part # HE721A1210. CH1 is the coil voltage, switched to ground by a transistor. CH2 is the SPST contacts switching the +12V to ground via a 1M resistor. This relay would be more than adequate for 50Hz chopper duty. Sounds like a quiet cricket chirping. Coil di/dt would need to be limited though at turn-on to prevent electromagnetic coupling into the high impedance switch circuit, however (note the spike in the "switch-on" scope photo).
EDIT: actually, that "electromagnetically" (or in reality, more likely capacitively) coupled spike might make the relay a dud for this application. A ~2V peak, that's around 2uA in the 1M. The 2V spike would probably integrate out over the 20mS period to a few 10's of millivolts - quite useless when the desired signal is uV! The spike as measured would need to be reduced by ~3 orders of magnitude to make this relay a decent high impedance chopper.