I sometimes hear about astronomers using an arc spectrum to calibrate observations. For example a "He-Ar arc spectrum". What is an "arc" in this context? I assume it's nothing got to do with angles (arcmin, arcsec, etc.).
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$\begingroup$ @ConradTurner, you could add some more details and make an answer out of your comment ! $\endgroup$– Py-serCommented Dec 1, 2015 at 14:53
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$\begingroup$ @LocalFluff See Conrad's answer. The arc refers to a discharge lamp. $\endgroup$– ProfRobCommented Dec 2, 2015 at 8:24
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An arc spectrum is one produced by a discharge lamp where the discharge is through ionised gas, in the case of He-Ar a mixture of Helium and Argon, which produces a predictable line emission spectrum.
They are often used to provide a calibration spectrum for spectrometers.
answered Dec 1, 2015 at 16:01
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$\begingroup$ An electric arc, in air, between two iron electrodes, is also used as a source of reference emission lines... $\endgroup$– DJohnMCommented Dec 5, 2015 at 5:56