I am doing an experiment for my lab work which include determination of value of Rydberg Constant using method of Interpolation. For this we first plot calibration curve for various wavalengths observed in spectromenter for Mercury Source and replace light source to Hydrogen Discharge Tube and then interpolate values of angles for H-lines in spectrometer window from calibration curve. Ideally we should see 4 emission lines for Hydrogen H-alpha, H-beta, H-gamma and H-delta lines. But for our source we can only able to see H-alpha (Red) and H-beta (Blue-ish-Green) lines. The possible reason behind this is that our hydrogen source is very dim (intensity of light is small). The voltage of 2000V is applied across the gas discharge tube. So what is reason behind absence of H-gamma and H-delta lines, why we were not observe these lines? If the problem is due to very small intensities of these lines, does increasing the voltage across tube to 2500V or 3000V will help? Does increasing voltage across tube will damage the tube? Is there any upper limit for voltage to be applied across Hydrogen Gas discharge tube? Can I find this upper voltage value in manual of tube?
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2$\begingroup$ If your tube is of commercial manufacture, get and check the data sheet for safe voltage levels. $\endgroup$– dmckee --- ex-moderator kittenAug 25, 2015 at 19:52
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$\begingroup$ Is checked with college lab, they don't have any manual for the tube. Is there any alternative method to find the safe voltage? I know trial and error will not be a good way. $\endgroup$– CrownedEagleAug 25, 2015 at 19:58
1 Answer
The hydrogen discharge tubes typically used in student labs are not designed for long-term use. After a couple of years, the tubes leak and air gets mixed with the hydrogen. This causes them to get dim and the weaker lines are almost impossible to see.
It has nothing to do with the power supply and everything to do with how new the tube it and how many hours of use it has seen.
You need a new tube. I know this from many years of running this experiment.
EDIT Addition: In answer to the question about a higher voltage, running at a higher voltage for a new tube might provide brighter lines, but will also cause impurity gases to show their lines. Higher voltages will also cause much shorter tube lifetimes (faster leakage due to higher temperatures at the penetrations for the wires). Running a higher voltage on an old, leaky tube will not provide the improvement you need on the weak lines. I've tried it before.
