Neon Lamp (5D40.50)
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Image Source [Wikimedia.org]
A neon lamp does not conduct below 80 V, but passes current easily above that voltage. It will continue to carry current down to about 60 volts once the current has been started.
The NE34 neon Lamp has a breakdown potential of about 90 Volts and is connected directly to a DC power supply. Turn the voltage up to about 90 Volts and the cathode will glow from the positively charged neon ions.
Once the discharge has been started, the voltage can be decreased to about 60 Volts before the discharge cuts out.
Reverse the leads to the DC power supply and the other terminal will become the cathode and glow.
Connect the neon lamp to a Variac and turn it up to about 90 Volts. Both terminals will appear to glow but they are really oscillating at the 60 Hz AC line frequency.
1) Neon Lamp [Wikipedia.org].