  We report on the effect of the back-action of a Single Cooper Pair Transistor
electrometer (E) on the measurement of charge on the island of a
superconducting box (B). The charge is e-periodic in the gate bias of B when E
is operated near voltages 2Delta/e or 4Delta/e. We show that this is due to
quasiparticle poisoning of B at a rate proportional to the number of
quasiparticle tunneling events in E per second. We are able to eliminate this
back action and recover 2e charge periodicity using a new measurement method
based on switching current modulation of E.
