166 
MESSRS. W. SPOTTISWOODE AND J. FLETCHER MOULTON 
I .—Definition and description of the sensitive state. 
It lias frequently been remarked that the luminous column produced hy 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. 
Tiie state in which the discharge is affected by the presence or approach of a conductor 
will be called the sensitive state; while that in which it is not so affected will be called 
the non-sensitive state. The former is the subject of the present inquiry. 
The attention of the authors of this paper was drawn to this form of vacuum dis¬ 
charge, partly by the striking character of the phenomena which it presents and partly 
by their own earlier observations which led them to connect it with an interruptedness 
or intermittence in the current. Although there are a great variety of methods by 
which the sensitive state can be produced, they all agree, as will presently be seen, in 
causing a rapid intermittence in the current. The experiments of Mr. De La Rue and 
others have furnished strong evidence for concluding that vacuum discharges are never 
continuous in the same sense that the flow of electricity along a metallic conductor is 
continuous, but that they are always disruptive and periodic with an extremely high 
rate of periodicity. Such being the case, it appeared desirable to subject to special 
examination a form of discharge in which periodicity (though much less rapid) is 
* Subsequent experiments Lave removed all doubt on this point. So long as tbe tube permits any traces 
of a luminous column to appear, this column can be rendered sensitive. 
