Mitch here is some data I took on M^2 apparatus showing the
negative resistance of a low pressure gas discharge along with a curve fit
of one set of data..
The figure on the left compares the volt ampere curves of a fixed resistor and two gas discharge tubes one with an i.d. of 1.0 mm bore the other 7.75mm bore. They were filled to a pressure of 5.0 mm.
Note the fixed resistor obeys Ohms law, V=IR i.e the voltage is directly proportional to current. For the top gas data V=80.9* I^-0.2409
The top three curves show "negative resistance" which is typical of arcs and low pressure gas discharge devices.
Here was a surprise to me.The He 7.74 mm dia. arc showed good voltage regulation for a current range 5 milliamperes to the limit of the supply 21 milliamperes it was about 110 volts per cm. Eureka a gas tube regulator/