VI. On the sensitive state of electrical discharges through rarefied gases

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It has frequently been remarked that the luminous column produced by electric discharges in vacuum-tubes occasionally displays great sensitiveness on the approach of a finger or other conductor to the tube. The exact effect of such approach varies considerably with the circumstances of the discharge. In many instances the luminous column is repelled; in others, and especially when the finger is brought into actual contact with the glass, the column is severed; and in the latter case, in addition to the luminosity previously present, there often appears proceeding from the interior of the tube, at the point where the finger rests, the blue haze which usually characterises the negative end of a discharge. In some cases the discharge is so powerfully affected that the well-known green or blue fluorescence appears on the side of the tube opposite to the point touched. The degree of sensitiveness varies between wide limits. Discharges frequently occur in which close observation is necessary to detect any trace of it, while others may be produced so sensitive that the magnetic action of a powerful electro-magnet is hardly more marked than the action which is due to it as a conductor. The condition in question does not appear to be confined to any particular gaseous medium or to any special form of tube; and it is in fact probable that in almost any tube sensitiveness may be produced by adopting suitable precautions. This state of sensitiveness may be exhibited by stratified discharges, but it is more commonly associated with those which show no clear traces of stratification. It is not however universally present in either kind of discharge.