How to safely demonstrate backfeeding AC
It seems like you are trying to simulate 60 Hz voltage that is induced or otherwise appearing in troublesome levels in cable television coax wiring. You should definitely not use something that is connected directly to a wall outlet. If you have a small variable transformer available, you could feed that with a 1:1 isolation transformer. Put a resistor in series with the output of that so that you have a high impedance source. You probably shouldn't use anything less than 25K ohms. See if that give the effect that you want to simulate.