ON THE SENSITIVE STATE OF VACUUM DISCHARGES. 
593 
phosphorescence" when a conductor is brought into contact with the tube is not very 
strongly marked, and is in fact often difficult to detect; but in these cases it will be 
found that the phosphorescence is highly sensitive to the approach of a conductor 
which is in metallic connexion with the negative terminal of the tube, a property 
which is quite as distinctive of the luminous phenomena of the sensitive discharges of 
which we have treated in our former paper as is their sensitiveness to the approach of 
a conductor which is not in connexion with any portion of the circuit. We can thus 
apply the term “sensitive state” to discharges through tubes of high vacua, even 
though the phosphorescence should constitute the main or even the only visible portion 
of the phenomena. 
Taking, then, this extended conception of sensitiveness, we find that it appears in 
tubes of high vacua under precisely the same conditions as in the cases with which we 
have dealt in our previous paper. A machine giving a continuous current produces a 
sensitive discharge when an air-spark is introduced; and so does a coil which does not 
give too much quantity. Indeed, it is in some respects easier to obtain a sensitive 
discharge in the case of tubes of high vacua than in that of tubes of low vacua on 
account of the great resistance they present, and the consequent need of considerable 
violence in the discharge if it is to pass through them. In the case of a coil, for 
instance, it is only the first and more violent part of the discharge that has force 
enough to penetrate into the tube, and consequently the discharge within the tube 
has often the sharp impulsive character necessary for sensitiveness, when in a tube of 
less exhaustion the discharge from a similar coil would be more prolonged and pro¬ 
bably non-sensitive. This property has, however, its disadvantages as well as its 
advantages. Many of the methods by which we succeeded in obtaining sensitive 
discharges in tubes of moderate vacua and therefore small resistance, are inapplicable 
to the case of tubes of high vacua where the resistance is necessarily very much 
greater. Such a method as the use of the wheel-break with the Holtz machinef 
would seldom if ever succeed in causing a current of any kind to pass through a tube 
in which the vacuum was very high. Before the machine had charged up sufficiently 
to give a current capable of passing through the tube the next division of the wheel- 
break would have come into contact with the platinum spring, or would have 
approached it sufficiently to induce the charge to adopt that path in preference to 
passing through the tube. 
There is therefore no need of an elaborate investigation to show that the sensitive 
* A full account of the phenomena due to the sensitiveness of the phosphorescence in the intermittent 
discharge and of the effects produced on it by a conductor in metallic connexion with one or other of the 
terminals of the tube will be given in the subsequent sections. It is not necessary here to do more than 
refer to the fact that changes can be produced in the phosphorescence by the approach of conductors 
which are either uninsulated or in metallic connexion with some part of the tube. The nature of those 
changes does not concern us at this stage, 
t Sec Phil. Trans. 1879, Part I., p. 170. 
