Geissler tube with hydrogen gas is not emitting light I have a Geissler tube that is connected to a Variac(AC power) and I am not getting any light. The gas that I am using is hydrogen gas. My Variac works as I checked it with a multimeter. The alligator clips are connected to the (copper or aluminum?) wires that serve as the electrodes of the tube. Any ideas of why this setup doesn't work? I have been told that AC should work for light emission. My setup is below:

 A: There are two problems with your setup.

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*First, in order for the arc in the tube to be initially formed, to “strike”, you need much more voltage than the Variac can supply: somewhere around 2-10 kV, much more than 120 V line voltage. (I'm guessing at the necessary voltage based on the size of your tube. The exact voltage is not critical due to the next point.) If you do not apply enough voltage, the tube resembles an open circuit — almost no current will pass and the tube will not glow.


*Second, once the arc does strike, the tube will then exhibit a much lower resistance, and so there must be a mechanism to limit the current passing through the tube (which can be as simple as a resistor or inductor, then known as a “ballast”), or it will overheat and be destroyed.
These are the same requirements as when operating more common gas-discharge tubes such as a “fluorescent lamp”, or a “neon sign” tube. I recommend you read about power supplies for these purposes, whether manufactured or made from components; there are a wide variety of designs. “Spectrum tube” is one name for a modern form of Geissler tube, and you can also obtain power supplies designed for precisely that purpose (though check if the dimensions of your tube are compatible).
