Intensity measurements in the arc spectrum of copper have been made for the quartets $a^4{D}^1-c^{4}D$, $a^{4}F-c^{4}D$, $a^{4}P-c^{4}D$, by using an electric arc at atmospheric pressure, and currents ranging from 1 to 19 amperes. It has been shown that the lines of these quartets may be divided into two groups, (a) those with initial terms $c^4{D}_{1,4}$ (sharp lines) and (b) those with initial terms $c^4{D}_{2,3}$ (broad lines), such that within each group the intensity relation of the lines remains practically constant for all current strengths. In comparison with group (a), however, the lines of group (b) increase their intensity rapidly with current strength up to about 12 amperes, beyond which they remain constant. The intensities approach nearer to the sum rule predictions for currents over 12 amperes, but even then there are some deviations. Nevertheless the similarity between the observed and calculated values supports the accuracy of the line classification.

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