Intensity Relations in the Helium Spectrum

Low-voltage arc in pure helium.—Inside a blown quartz tube, electrons from a tungsten spiral were accelerated into the equipotential space inside a hollow copper anode. Unusual purity was obtained by heating the anode red hot by high frequency induction to remove gases, in addition to other precautions. Minimum operating potential was found to be 21.5 volts, just above the resonance potential. The relative importance of the ionizations at 20.5 and at 25.5 volts depends on the pressure and current density; after the current has risen to a high value the 25.5 point is obscured, indicating probably a decrease in the proportion of normal atoms under the increased radiation. Change in relative intensity of arc spectrum lines with voltage was studied for 25 to 85 volts, using the neutral wedge method of Nicholson and Merton. The crossed orbit lines $\lambda 4437 \mathrm{}(1P-4S)\mathrm{}$, $\lambda 4387 \mathrm{}(1P-4D)\mathrm{}$, $\lambda 4922 \mathrm{}(1P-3D)\mathrm{}$, $\lambda 5016 \mathrm{}(1S-2P)\mathrm{}$ all increase with voltage, though not uniformly; while the coplanar lines $\lambda 4713 (1\mathrm{}\pi -3\mathrm{}\sigma )$, $\lambda 5875 (1\mathrm{}\pi -2\mathrm{}\delta )$, $\lambda 4471 (1\mathrm{}\pi -3\mathrm{}\delta )$ all diminish to a minimum at about 50 volts.