Stark effect in iron and the contrast with pole effect

In the operating conditions of the iron arc there are a number of influences which may measurably affect the universally accepted standards of wave-lengths. Since this phenomenon appears to have some complexity, it is important to learn what part may be played by pure Stark effect. Already it is known that an increase of pressure within the source causes small shifts in the wave-lengths of most iron lines (pressure shift), which according to Babcock (1928) is always toward the red. This effect appears to be different from the so-called pole effect which consists of a relative change of wave-length at the pole as compared with that at the centre of the arc. For it is found that in the latter effect the shift is toward the red for some lines and toward the violet for others. In recent years the pole effect has been rather generally accepted as a pure Stark effect due to electric fields developed by the relatively large drop of potential near the pole. This view has indeed been taken as a result of earlier examinations of the Stark effect for iron in which the displacements of a few lines were reported to have the same sign as their pole effects.